There would have been four in the Coalition's tally but for sacked Defence Minister David Johnston deciding to knock back Prime Minister Tony Abbott's previously unreported offer to become Australia's next High Commissioner to New Zealand.

"He's already rejected it. He's very happy representing the people of Western Australian in the Senate," a spokeswoman for Senator Johnston told Fairfax Media.

But with several other key jobs as Australia's ambassador to important countries about to fall vacant – including the US, China and Japan – former politicians and key political advisers are in the mix for a top posting.

"Tony Abbott has made a notable number of political appointments in a relatively short period of time," said Russell Trood, a Griffith University professor of international relations and former Liberal senator from Queensland.

Both sides of politics have a tradition of raiding the political ranks to create what Professor Trood described as "busy amateurs" in diplomatic roles, with top-line posts such as Washington and London regularly filled by people close to the government in Canberra.

"The assumption is they can speak to the politicians there because they understand the language," said Patrick Weller, chair of Governance and Public Management at Griffith University.

"The diplomats don't like it, because each job that goes to a politician is one less job for them," said Professor Weller.

Labor sent former South Australian premier Mike Rann to London in 2013, only for the Abbott government to appoint him Ambassador to Italy a little over a year later to make way for former foreign minister Alexander Downer.

John Howard made his one-time leadership rival Andrew Peacock the country's top envoy to Washington in the 1990s, as did Kevin Rudd for Kim Beazley in 2010.

There is little sign the Abbott government will return Mr Rudd's political favour to cross the aisle and appoint conservatives to plum posts (he sent Brendan Nelson to Brussels and Tim Fischer to the Vatican).

Former Victorian Labor premier Steve Bracks never got to take up a job as New York consul general, dumped in 2013 by the incoming federal Coalition government in favour of ex-South Australian Liberal senator Nick Minchin.

But Mr Beazley's term as ambassador in Washington has been extended by the Coalition, and one-time Democrats leader Natasha Stott Despoja given the roving job of Ambassador for Women and Girls.

A replacement for the Washington job is the subject of the most heated gossip, with Mr Beazley expected home at year's end and former Treasurer Peter Costello one of several names persistently rumoured to replace him.

Mr Abbott's national security adviser Andrew Shearer, a former diplomat who also worked in John Howard's office, is another name mentioned.

Mr Abbott's international adviser from opposition, Mark Higgie, also a former diplomat, was sent in 2014 as ambassador to Brussels.

Paul Keating as prime minister also sent adviser Don Russell to the gig in Washington after Bill Clinton became US president.

There has also been a history of what Professor Trood called "patronage" appointments.

"It sometimes suits Prime Ministers to offer political colleagues another career opportunity in the context of a ministerial reshuffle," he said.

Former Queensland senator Brett Mason was dropped in December as parliamentary secretary for foreign affairs in Mr Abbott's cabinet reshuffle, and in April announced as Australia's ambassador to the Netherlands.

Senator Johnson was the other notable casualty, with Mr Abbott saying at the time of both men: "I'm confident that both Senator Johnston and Senator Mason will continue to serve the Government in various capacities in the months and years ahead."

Mr Abbott has also appointed former federal police chief Tony Negus as high commissioner to Canada.

A count of non-career appointments to what are called "head of mission" jobs shows a rate presently 50 per cent higher than at any time during the modern history of Australia's diplomatic service.