Donald Trump winning the United States election would be "dangerous" for the whole world and even talking about the prospect is "scary", Greens leader Richard Di Natale says.

Key points: Senator Di Natale calls for a revaluation of Australia's alliance with the US if Mr Trump becomes president

Senator Di Natale calls for a revaluation of Australia's alliance with the US if Mr Trump becomes president Mr Trump is "dangerous for the global community", Senator Di Natale says

Mr Trump is "dangerous for the global community", Senator Di Natale says Senator Di Natale says he agrees with Mr Shorten's earlier comments criticising Mr Trump

Senator Di Natale backed Opposition Leader Bill Shorten's assessment of Mr Trump, whom the Labor leader described as "barking mad".

Mr Shorten made the comments on commercial radio in Darwin last week, but later insisted he would be able to work with Mr Trump should they both be elected to office.

Mr Trump has secured enough convention delegates to secure the Republican nomination for the presidency.

Prime Minister Malcolm Turnbull said Mr Shorten's comments were contrary to Australia's national interests.

However, Senator Di Natale told the ABC's Insiders program he agreed with Mr Shorten's comments, and the thought of a Trump presidency scared many Australians.

"It's a statement of the bleeding obvious, and I think it's worse than that," he said.

"I think he's dangerous for the US and dangerous for the global community, and you just need to look at those clashes that we have seen recently between anti-Trump and pro-Trump supporters."

Sorry, this video has expired Bill Shorten defends criticism of Donald Trump

Senator Di Natale said if Mr Trump was to become president of the United States it would be in the national interest to redefine the terms of Australia's alliance with the US.

"We think it should be done. Particularly when you consider the fact that we are even talking about a Trump presidency scares the daylights out of most people," Senator Di Natale said.

"I think it is absolutely in our national interest to redefine the terms of our alliance and not to commit troops blindly to international conflicts in the way we have done in the past.

"We are the only country that's followed the US into every international conflict since World War II, including some major strategic blunders.

"I think the sign of a mature relationship is one where you can stand up to a partner and tell them when you think they have got it wrong.

"We haven't done that, to our own detriment and to the detriment of the international community."