Bill Shorten reckons Donald Trump is barking mad.

But with the nation’s top job in his sights, he isn’t about to say it on camera.

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The opposition leader made the assessment of the presumptive United States Republican Party candidate during a local FM radio interview in Darwin, drawing a swift rebuke from Prime Minister Malcolm Turnbull.

He was reluctant to repeat it later for national television.

“I have already said it,” he insisted when pressed by reporters to repeat his comments on Friday.

Mr Shorten, however, was prepared to defend his right to express opinions about Mr Trump, highlighting he was a man who believes Mexicans are “killers and rapists”.

“People ask me what you think about Donald Trump,” he said. “Will I just stand here and tell you nothing? No.”

In any case, Mr Shorten reckons his opinions are in line with former Liberal prime minister John Howard who isn’t copping the same criticism from the coalition.

And his barking-mad evaluation won’t hurt US-Australia relations, even if he makes it to prime minister and Mr Trump is installed as president.

“We will maintain the American alliance no matter what,” he said.

Prime Minister Malcolm Turnbull wouldn’t provide an assessment of Mr Trump, preferring to leave that to the American people.

But he criticised Mr Shorten for his “ill-judged remark”, saying it would be seen as offensive to Americans.

“You can imagine how Australians would feel if an American president were to describe one of our prime ministerial aspirants as barking mad,” he told reporters in Brisbane.

“You can imagine the ill-will and resentment that would create in Australia.”

Mr Turnbull expects Americans to make a wise decision when they vote later this year.