Former US president Donald Trump has lashed out at former Australian prime minister Kevin Rudd, characterising him as “not the brightest bulb”, giving rise to concerns about the strength of future US-Australia ties.

In an interview with former far-right British politician Nigel Farage on London’s GB News channel, the second-time presidential hopeful responded to years-old comments made by Rudd.

Before his appointment as US ambassador, Australia’s 26th prime minister was a vocal critic of the Republican president, calling Trump a “traitor to the west” and “the most destructive president in history” who “drags America and democracy through the mud”.

Trump has now returned fire, issuing a warning to Rudd.

“I don’t know much about him, I heard he was a little bit nasty,” Trump told Farage. “I hear he’s not the brightest bulb ... if he’s at all hostile, he will not be there long.”

The opposition was quick to pounce on Trump's comments during Question Time on Wednesday.

Prime Minister Anthony Albanese was not having it, describing the line of questioning as a "cheap shot" and accusing the coalition of politicising the ambassador role.

"They decide to politicise Australia's representative in Washington DC," Albanese told parliament.

He said that was never done by Labor when former Liberal politicians, including Arthur Sinodinos and Joe Hockey, were appointed to that position.

"Because one of the things that I have never seen happen before in 28 years in this chamber is an attempt to politicise Australia's representative overseas in an important nation such as that," Albanese said.

Opposition Leader Peter Dutton said Rudd needed to repair the relationship with Trump.

"It is in our national interest to make sure that the Ambassador to the United States, our most important strategic alliance partner, is successful and we will do everything we can to support him," he said.

The US presidential election in November is expected to be a rematch between Democrat President Joe Biden and Trump.

Biden's approval rating has continued to slip, while Trump is contending with multiple criminal charges.

Nationals leader David Littleproud warns Rudd's past comments could have consequences for Australia's alliances and defence deals if Trump wins the election.

"We're going to have a very difficult time in negotiating with the new administration, particularly with things as important as AUKUS," he said.

But Foreign Minister Penny Wong defended her former colleague and said Australia would keep Rudd as ambassador.

"Mr Rudd is a very effective ambassador," she told reporters.

"His experience and skills mean he will be able to work closely with whomever is elected by the American people."