Australia insisted Wednesday that its ambassador to Washington was doing a "good job" after Donald Trump suggested he was "not the brightest bulb" and could be expelled from the United States.

Trump lashed out during a TV interview at former prime minister-turned-diplomat Kevin Rudd, who has been critical in the past of the bombastic former US president.

When prompted to comment on Rudd, Trump said he had heard he was "a little bit nasty".

"I hear he is not the brightest bulb," Trump said, adding that "if he is at all hostile, he will not be there long".

Trump is the presumptive Republican presidential nominee for November's US election, and polls show him with a narrow edge over incumbent President Joe Biden, with the race expected to be very close.

Australian Foreign Minister Penny Wong said Wednesday that Rudd would remain in the role and had the government's confidence.

"Mr Rudd is a very effective ambassador. He is recognised across this parliament as doing an excellent job in advancing Australia's interests in the United States," she told reporters in Canberra.

"I point you in particular to the phenomenal amount of work which has been done on AUKUS in the period that he has been ambassador" she said, referring to a security pact between Australia, Britain and the United States.

"He's been active in engaging with members of Congress on both sides of politics," Wong continued.

She said that Rudd's background, including his time as a foreign minister, meant he had the experience and skills to work with whoever was elected US president in the upcoming election.

Rudd, who took up the diplomatic role in March 2023, had previously described Trump as "nuts", "the most destructive president in history", and a "traitor to the West".

A fluent Chinese speaker, Rudd's tenure has focused on a deal to allow Australia to develop nuclear-powered submarines and a host of other advanced US weaponry.

State Department spokesman Vedant Patel, asked about Trump's remarks, called Rudd "an important partner" in Washington.

"I know folks around this building have enjoyed working with him and will continue to do so," Patel told reporters.

Democratic congressman Joe Courtney said on X, formerly Twitter, that Rudd's appointment had strengthened relationships between the countries.

"He is respected & admired by legislators on both sides of the aisle -- a rare feat in DC," he wrote.

Trump was also asked during the GB News interview about whether Prince Harry should receive "special privileges" if authorities found he had lied about taking drugs on his US visa application -- something that normally leads to deportation.

"If he lied, we'll have to take appropriate action," Trump warned.