Former prime minister Kevin Rudd has launched a scathing attack on Donald Trump, declaring the American president to be "nuts" and warning that China could step into a potential regional power vacuum.

"I think the general consensus amongst anyone concerned with a public policy process, domestic or international, thinks he's nuts," Mr Rudd said on the ABC's Q&A on Monday night, in response to a question about US voter support for the President.

Mr Rudd, now based in the US but in Australia promoting his memoir, said he had remained silent for the first six months of the Trump presidency.

But, he said: "One small example … I spent a huge amount of time as PM getting America to be accepted as a member of something called the East Asian Summit. The whole idea was to get the Americans into regional diplomatic institutions concerned about long-term regional security. Prior to that they were outside.