Tim Williams set off from Port Kembla boat ramp for a fishing expedition on Saturday morning, throwing a line in by the Wollongong FAD (fishing attraction device), Bellambi Reef, and then near Bass Islet.

It was the latter location where the majestic creature attached its mouth to the back of the ski and "rubbed his chin", a creature that is usually found in tropical waters.

"I'm out there all the time and I see whales regularly, dolphins and seals regularly. It's rare that they come up to the actual jet ski," Mr Williams told the Mercury.

"It might have been there for a little bit but I was focused on the task at hand - trying to catch a snapper.

"I eventually looked to my left and I just could not process what I was seeing, because I knew this was a whale shark but it didn't make sense, I thought 'It shouldn't be here'."

The avid fisherman said he had dived with whale sharks in the past when holidaying in Thailand and knew it was unusual to be off the East Coast. He estimated this one to be at least twice the length of his jetski (which is 3.5 metres).

Whale sharks are a threatened species and more commonly found off the north coast of Western Australia though sightings have been reported off the east coast (including as far south as Eden), according to Australia's Department of Climate Change, Energy, the Environment and Water (DCCEEW).

The carnivore can range in length from just over five metres but some have been known to reach up to 20 metres, and live around 70 years in the wild, according to Australian Geographic.

There have also been sightings in waters off India, the Maldives, South Africa, Belize, Mexico, the Galapagos Islands, Southeast Asia and Indonesia.

Earlier in February a whale shark was reportedly spotted by fishermen off K'gari (formerly Fraser Island) in Queensland, while Mr Williams believes it possible to be the same one he saw.

He said the encounter was truly "special" and he was grateful to capture it on film once he got his breath back.

The only other fishing encounter that has come close in his eyes, he said, was when two whales popped up near his ski and circled playfully - also captured on video on his YouTube Channel @TimsLifeOnTheCoast.

"[They came] up relatively close to it, but not too close," Mr Wiliams said.

"These other whales were the size of a bus, so that's pretty impressive when they're right in front of you. This [whale shark though] is by far the most special."