One of the Turnbull government's most senior ministers, Christopher Pyne, has waded into the US presidential contest, saying the popularity of Republican frontrunner Donald Trump is "terrifying" and is making American democracy look "kind of weird".

It is unusual for Australian politicians to criticise US presidential candidates during a campaign.

Mr Trump - a businessman who has made vitriolic attacks on immigrants, threatening mass deportations and proposing to build a wall on the Mexican border - is far ahead of his nearest rival, Ted Cruz, in the race for the Republican nomination. This week the businessman added Florida, Illinois and North Carolina to his primary tally.

Protests outside Mr Trump's rallies have sparked violence in recent days.