Sometimes the best attacks are the most subtle.

Tony Abbott's defence of US presidential candidate Donald Trump has elicited a snarky response from Prime Minister Malcolm Turnbull, who cast doubt on whether his predecessor was familiar with all of Mr Trump's policies.

The former prime minister came to the aid of the Republican nominee on Thursday, telling conservative broadcaster Paul Murray that Mr Trump's policies were "reasonable enough" and his supporters "decent people".

Mr Turnbull, who has described Mr Trump's remarks on women as "loathsome", declined to enter the fray on Friday, reiterating that the US presidential election was a matter for the American people, not Australian politicians or parliaments.