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.