Prime Minister Malcolm Turnbull says income inequality in the US is responsible for the rise of Donald Trump, acknowledging the outlandish billionaire has the Republican nomination "in the bag".

Mr Trump will be the presumptive nominee at the party's July convention, following an emphatic victory in the Indiana primary which forced rivals Ted Cruz and John Kasich out of the race.

Mr Turnbull said anger over real wages, which have shown little to no growth in the US for decades, contributed to the New York businessman's success.

"It does look like he's got it in the bag," the Prime Minister told Triple M radio. "There is a real sense of disappointment in middle America with the very slow growth in middle incomes.

"Income equality is a big issue in the United States, we have much more equality in incomes in Australia because we have a much better targeted social welfare system.

"But there are a lot of tensions there and I think support for Trump is clearly evidence of that."

Much has been made of Mr Trump's support among working class Americans left behind by economic change and disillusioned with the Republican Party establishment. But exit polling from state primary contests shows his base is broad and varied: 43 per cent of his backers had college degrees or postgraduate qualifications, and 34 per cent earned more than $US100,000 a year, The Economist reported.

The presidential race will now take a different tack, with Democratic frontrunner Hillary Clinton all but assured of taking the nomination. Mr Trump has taken to calling her "Crooked Hillary" in a sign of the invective to come.

Mr Turnbull conceded the brash businessman's apparent authenticity may be part of his appeal - he seems beholden to no entity or custom. But he predicted Mr Trump would have difficulty pivoting back toward the political centre, as candidates inevitably do once they have clinched the nomination.

"Making that switch is always very challenging and I think that's the challenge Donald Trump will have," Mr Turnbull said.

He said Australia's relationship with the US had been fortified over generations and Canberra would continue to enjoy a strong friend and ally in Washington regardless of who won the presidency in November.

"It's a very dynamic, colourful electoral system they have in the United States. We observe it with admiration sometimes, with consternation at others," Mr Turnbull said.