He enlisted in 1995, just as Israel was experiencing an unprecedented wave of suicide bombings launched by Palestinian group Hamas. A number of experiences over the next two years left him profoundly shaken psychologically – notably the deaths of army friends in a helicopter crash in 1997, and a car crash in Egypt that claimed the lives of two British friends to whom he was close. He returned, troubled, to Melbourne to finish his law degree, then secured a position in commercial law firm Deacons, where he was well-liked.

After five years, Zygier decided again to act on his passion. He moved to Tel Aviv, where, in 2003, he was recruited by Israel’s biggest and most prestigious law firm, Herzog, Fox and Neeman. It was here that he was first approached by the Israeli spy agency, the Mossad, and asked if he’d like to consider doing “some interesting work”.

In mid-2007, two-and-a-half years into his operational career with the Mossad, Ben returned to Israel to work in Tel Aviv. He moved out of the more active Caesarea department and into the bigger, more bureaucratic part of the Mossad called Tsomet. This meant he was desk-bound and faced work that was more analytical and potentially less exciting than his duties overseas. I am not able to establish whether Ben saw this as a demotion, and whether it was intended as such by the Mossad, or if it was simply part of the normal career progression within the agency. Regardless, it was reported that this development was frustrating for Ben, a blow to his aspirations.

In Ben's personal life, however, important things were happening with his wife, Haya. For the first time in the couple's married life, Ben was based in Tel Aviv. They'd set themselves up in the northern district of Raanana, one of the city's better commuter suburbs. As I drive into this neat, manicured suburb, I try to imagine what Ben and Haya's life was like here. I move slowly down a street elegantly lined with blossoming jacaranda trees, aware that this is one of those rare places in Israel where the streets are uniformly clean and most of the buildings look refreshingly contemporary.

Ben and Haya's first child, Naomi, was born here in November 2007. By this stage, after long aspiring to serve, Ben had already experienced Israel's army and the Mossad. Once Naomi was born, he may have felt he would never again achieve such elite, frontline roles, though he also may have felt ambivalent about this. "Your 20s is an extremely crazy time [in] many people's lives, and people change tremendously," says one friend of Ben's, alluding to the particular combination of confidence and frustration that seemed to mark Ben's life.

Sometime after he became a father, he went through a crisis. The memory of the deaths of his fellow soldiers a decade earlier, the pressure of a life of espionage and the strain of taking care of a child all likely took their toll. The Mossad had made a "big mistake", Ben's friend Lior Brand told an Israeli newspaper in March 2013. "There's very painful mistakes around it. They took the wrong guy. He couldn't deal with it." Brand, who had served in the Israeli Air Force, said, "Ben was not cut out for pressure like that. Most people are not cut out for this kind of stuff. I wasn't cut out for it."

The pressure Ben was under led him to make several suicide attempts. From what I'm told, one was in early 2008, possibly in Israel, and involved him "walking out into traffic", according to one friend. It has also been hinted to me that Ben attempted to take his own life the following year. It's difficult to confirm exactly what happened. Who knew about the attempted suicides and what help Ben received are some of the most closely guarded details of his life. One trusted source tells me that by the time Ben was in prison, his wife and parents knew about these incidents. But surprisingly, judging by the court documents, it seems the Mossad and the prison service did not.

It's my understanding that Ben did not tell his employer what had happened, but that general personal issues were a key reason why he was granted special dispensation to return to study in Australia. In mid-2008, he applied for an Australian government-funded loan to help pay for a postgraduate management degree in Melbourne the following year. Perhaps the Mossad was beginning to acknowledge what Lior Brand would point out years later: that Ben "was in over his head. They put the wrong guy in the wrong situation."

It is unsurprising that at this time of great psychological pressure, Ben appears to have made a big mistake. What I discover about Ben's alleged error, however, differs significantly from what has been previously reported by the print media in Europe and Australia. The media reports said Ben was disappointed to be given the desk job in Tsomet, frustrated at being rotated through another part of the organisation and desperate to prove he could do more in the field. It was reported that Ben was crushed by the move back to Tel Aviv and felt he was perceived by his colleagues as having failed in his field work in Europe - he was "neither particularly bad nor particularly good, but mediocre", according to a source.

The reports' central allegation is that in 2008, Ben decided to prove his worth by securing a trophy, and that without telling anyone at the Mossad, he flew to Eastern Europe. On this trip, he reportedly established contact with an associate of Israel's Lebanese enemy Hezbollah, in order to groom him as a double agent. But he was allegedly outsmarted while trying to prove his Mossad credentials, and ended up passing on classified information that exposed two of Israel's own double agents in Lebanon. The media stories asserted that these Israeli sources were arrested and jailed by the Lebanese authorities. They also claimed that this chain of events was revealed by Ben under interrogation by Israel's domestic intelligence agency, Shin Bet, in 2010.

The Australian media story has clear implications: Ben's motives were good, but he was incompetent and, most importantly, he'd done something outrageous, utterly beyond the bounds of what was acceptable. This perspective seemed to explain his suicide. It also implied that Ben lacked the fundamental skills needed for frontline intelligence work. The problem for me isn't that some of these elements might not be true, but that the story is so neatly tailored to serve the Mossad's interests.

The journalists tell me they were essentially told that Ben was a fabulist, an exaggerator willing to act on his own illusions. "And that was a big weakness, a big personal character flaw," one of them says. "I had been told that when he [Ben] was in Melbourne, he was talking up his experiences in the IDF [Israel Defence Forces], [saying] he had been involved in some battles and that he had been injured ... But that was an exaggeration." They are right about the exaggeration, but I believe they are wrong about the central allegation concerning the trip to Eastern Europe.

Later, a source in Tel Aviv tells me that the story published was wrong. I am surprised, but then an Australian government source gives me the same assurance. My informants insist that none of this supposed freelancing by Ben in Eastern Europe ever occurred, and that the decision to return to Tel Aviv in 2007 was driven solely by his desire to live with his wife and prepare for their first child.

I also have to weigh up what I have been told by Ben's family, friends and workmates. He is not described as someone who betrayed confidences or lied about his exploits, not by any of the people I speak to who knew him in the last years of his life. One friend says that while Ben was a bragger, he was not someone who made things up. Yet that is precisely the image that appears to have been embraced by many of the journalists who reconstructed his life after his death. "Arrogance is the wrong word," says a former law firm colleague of Ben's. "There was a great deal of self-confidence and a very buoyant character, but to hear or to read some of the stories now, [it] is all a bit of a surprise. There must be more to it because his character was not that way inclined."

But much of my research agrees with the suggestion in many of the published stories that Ben's boasting was the ultimate cause of his downfall. I am never told he went as far as flying unauthorised to another country to conduct freelance espionage. That event, I am explicitly told, did not happen. But Ben's willingness to mouth off was a problem. No one suggested Ben did this maliciously, or that his talkativeness came from a desire to further his own Mossad career. Ben got into trouble simply because he just didn't know when to shut up.

During his and Haya's final year in Melbourne in 2009, when they lived in an apartment in Caulfield, Ben confided little in friends and family about whether he would continue his career with Mossad, although he still received a salary from the agency.

Instead, it seems, he spoke volubly with his fellow students at the nearby campus of Monash University. Although his classmates were unfamiliar to him, he was good at making friends with strangers - some of his friends tell me he was better at talking with strangers than he was at maintaining relationships with people he knew well. It was this friendliness towards other students, which at first seemed like Ben's usual willingness to mix with others, that tipped into risky behaviour.

Ben's mistake was a simple one and lacked the determination and intent that has been suggested in the media. In Israel, the term used for Ben's crime is hasood, "the secret". More tantalisingly, it is sometimes described as livat hasood, or "the heart of the secret". Put simply, Ben said too much to the wrong person at the wrong time.

There were a number of overseas students in the management course, including people from Malaysia and Saudi Arabia. Significantly for Ben, there was also a businessman who had links to Iran, and it was this Iranian who played a key role in his downfall. My understanding is that Ben began talking about things he should not have ever discussed, not only telling people that he used to work for the Mossad but divulging details of the operations against Iran in which he'd been personally involved. It seems incomprehensible that someone who'd received Mossad training and was so committed to Israel would do this. He must have understood it was dangerous to talk about such things. Yet two sources tell me that's what Ben did in conversations at the university, including with the Iranian businessman. This man is described to me as being "unscrupulous", which makes Ben's willingness to disclose important facts to him even more surprising.

Ben did not have a senior role in the operations that targeted Iran. But it didn't matter. He had worked for a Milan-based satellite company that had operations across Africa and the Middle East, he had been to Tehran, and he knew plenty about what the Mossad was trying to do. His loose talk was a serious problem, especially as the program of assassination of Tehran scientists was then near its peak. Even just the leaking of Ben's name and the name of the Italian company could have potentially been disastrous. Simply tracking his movements in and out of Tehran using his passport details, determining whom he had met with, could have exposed many Iranians to dangerous scrutiny as would-be Israeli informants. The sources who confirm this version of Ben's central mistake will not spell out the details of what he said, but they tell me it was significant enough to get his erratic behaviour flagged in Tel Aviv, and for the Mossad to begin an investigation.

After cross-checking various facts and details, I now believe the Iranian businessman knew what he was hearing was valuable, and relayed the information back to Iran. Tehran's security services started talking with the Iranian about the Mossad agent, and the details revealed by Ben then became part of the electronic traffic between Tehran and a European capital city - probably in an effort to find out more about what Ben had done in Europe. This traffic was then picked up by the huge surveillance network operated by Israel's Unit 8200.

It comes as no surprise, then, that when Ben blabbed in Melbourne, the Mossad heard about it in Tel Aviv via Tehran. And Ben's chatter was serious enough to bring down the full weight of the security state on his shoulders.

In December 2009, Zygier received a phone call from Fairfax Media's then Middle East correspondent Jason Koutsoukis, who asked him outright if he was a Mossad spy. Koutsoukis, acting on an unexpected tip-off from Australian intelligence, said later that Zygier's surprise and denial sounded genuine. At this time, Zygier was also receiving ever more urgent calls from the Mossad requesting his return to Israel.

In January 2010, a senior Hamas military commander, Mahmoud al-Mabhouh, was assassinated in a Dubai hotel. Allegations that the assassins, widely believed to be Mossad agents, used fraudulently obtained passports from countries including Australia led, in May, to the expulsion of an Israeli diplomat from Canberra. For the first time, the Australian government directly accused the Mossad of using counterfeit Australian passports in its overseas operations.

Zygier returned voluntarily to Israel with Haya, who was pregnant again, and Naomi. On January 31, 2010, Shin Bet took him to an interrogation room in Tel Aviv. On March 7, he was transferred to Ayalon Prison in nearby Ramla. Incredibly, most inside the Australian government remained oblivious to his plight.

Ben's alleged crime was so significant that almost no one on Ayalon's staff, from the most senior man in the prison, the deputy commissioner, down to the junior warden who brought him food, was told much about him at all - not his name, his age, what he was charged with, or how long he'd been held. Talking with him was discouraged. Ben was not even given a prisoner number. This meant that when staff wanted to communicate with each other about this inmate, rather than a shared computer file that everyone could access, these wardens relied mainly on what one guard called "from my mouth to your ears". Nothing was to be written down. Everything about Ben was "shoo-shoo", Hebrew slang for "secret".

The Mossad and Shin Bet are the guardians of Israel's most critical secrets, and Ben had been accused of spilling at least one - related to operations against Iran - into the outside world. He had breached the crucial article of faith that governs every intelligence agency: secrets must remain secret. And so Ben was held under the tightest security that Israel could muster. But it was security that compromised his rights and his care.

Ben saw his lawyers regularly and a psychiatrist once a month between August and October; he seemed to have coped pretty well with being locked up in prison while his second daughter was born, in July. In November, however, he plunged into a profound personal crisis. He and Haya had been struggling to maintain their relationship amid the strain of separation and the pressure of keeping his situation secret. Haya's prison visits were difficult for both of them, and among their close circle of family and friends, some feared that the marriage could not withstand Ben's long-term incarceration.

By November, he was falling apart, and his final secret, which he'd managed to keep hidden despite the pressures of jail, could no longer be kept. After nine months in custody, he finally admitted that he'd previously tried to take his own life. The revelation raises a host of questions about why the prison psychiatrists had failed to uncover such a vital piece of information in Ben's previous months in Ayalon. Perhaps the signs were too difficult to pick up, though this seems unlikely.

Ben woke early on what would be his last day alive, nervous about an upcoming visit by Haya. He knew she was struggling. She'd had to re-establish a home in Israel at short notice and work to calm the children, in particular helping her older daughter adjust to life without her father. Even so, he did not anticipate the stunning news she was about to deliver.

According to court documents, Haya was escorted into Ben's cell by a junior intelligence officer at 11.10am on December 15, 2010. The cameras in his cell captured every moment of Ben's next-to-final conversation with his wife as their five-month-old daughter watched on.

But they could not record Haya's exact words or how Ben felt as it dawned on him that his world was radically changing yet again. Nor were the guards listening in, so they couldn't register the full weight of Haya's announcement that she wanted a divorce.

For Ben, the news was at first shocking, and ultimately devastating. Assurances were given about maintaining contact with the children, but the family he thought would be waiting for him when he eventually left jail would no longer be there. Court records revealed that Ben could be seen on the camera footage crumpled with grief. When Haya walked out of his cell, also now in tears, she was unaware it was the last time she would see her husband alive.

Ben, deeply upset, contacted the person he always rang, his mother Louise, who was at home in Melbourne. He passed on the devastating news that Haya was leaving him, unable to deal anymore with his situation. He also told his mother he was considering knocking back the plea bargain he had been offered and, instead, fighting back against the system, even if this could potentially increase his final jail term.

According to a family friend, during this conversation, Ben uttered the words, "I'll see you in the next life." It was enough to prompt a rapid response from his mother. She sent an email to Ben's lawyer in Israel, Moshe Mazur, writing that she feared Ben would overdose on the sleeping pills he had told her he was stockpiling. Mazur then quickly called Ayalon's command centre and asked to speak with his client, and he spent the next hour talking to Ben. It was another in a series of calls that expressed concern about Ben's welfare to those in charge of the prison. By this stage, the callers believed they had raised enough red flags to ensure he would be properly cared for.

At 6.05pm, Ben switched off the lights in his room and turned on his TV. Camera footage of the cell at that time showed him putting something on a chair near his bed, but the poor-quality image made it difficult to discern. It would turn out to be a bedsheet that he had torn into two pieces. A short time later, he turned off his TV.

At 6.54pm, Ben got up from his bed, turned on the TV again and went into his bathroom for the last time. The feed from camera 116 showed Ben standing in the shower stall, mostly in shadow but vaguely illuminated by the glow coming from the TV. But this was the camera in Ben's cell whose images could not be relayed to the command centre, just to the adjacent guardroom. So even if the officers in the command centre had been watching the feed from Ben's cell, they would not have seen what transpired.

The footage from camera 116 was indistinct, but it was viewable enough for the investigating judge to describe what happened next: "Despite the extremely poor visibility, when you focus on camera 116 you can see [Ben] doing something for 20 minutes in the location where he hanged himself, and several times you can see the silhouette of a man moving up and down in the same location. The footage is blurry and dark, but since the TV set was on, its light helps to show what is happening. Despite the darkness, you can see a man's shadow in the area of the hanging for several minutes without moving. If you view the footage meticulously, you can even see his death throes."

At 8.13pm, a junior officer noted in the logbook of cell 15 that the inmate was not answering his intercom. He did not enter the cell but summoned a master sergeant, who arrived six minutes later and stepped inside. Turning on the light, he saw Ben in the shower stall, unmoving, the sheet tight around his neck.

An ambulance was called as efforts were made to resuscitate Ben, but it was too late. The vibrant young Zionist from Melbourne, a man who had been keen to prove he belonged among Israel's elite, had taken his own life.

Edited extract from Prisoner X, by Rafael Epstein ($30), published by MUP next week.

Lead-in photograph by AFP.