Donald Trump is a "terrifying" threat to democratic order thanks to his "creepy" embrace of autocrats such as Vladimir Putin, according to former prime minister Malcolm Turnbull.

In a stark warning about the prospect of Mr Trump returning to the White House, Mr Turnbull told the ABC's Q+A that Mr Trump and parts of the Republican Party were "sympathetic" to the Russian president and "no longer committed to democracy as we understand it".

"Are we going to find ourselves not dealing just with two autocracies in Russia and China, but what is Trump's America going to look like?" Mr Turnbull asked.

Mr Trump is almost certain to claim the Republican nomination to run for president against Democratic incumbent Joe Biden in November. He has led Mr Biden in some national head-to-head polls.

Mr Turnbull served as prime minister for part of Mr Trump's first term in office and said he observed his "awe" for Putin.

"When you see Trump with Putin, as I have on a few occasions, he's like the 12-year-old boy who goes to high school and meets the captain of the football team. My hero!

"He is attracted to dictators and tyrants like [North Korean leader] Kim Jong Un and [Chinese President] Xi Jinping and he threatened to undermine or pull out of America's longest standing alliances."

Trump 'threat to Australian security'

Amid a reckoning of the Coalition's time in power, stoked by the ABC's Nemesis series aired earlier this month, Mr Turnbull used Q+A to take fresh swipes at the Liberal Party and its policies.

He said the man who replaced him as PM, Scott Morrison, was wrong to dismiss the threat Mr Trump posed to Australia.

In an exit interview with Nine newspapers ahead of his final parliamentary speech on Tuesday, Mr Morrison said another Trump presidency "doesn't pose any concerns in terms of the impact on Australia's national interests".

Mr Turnbull's response? "If Scott Morrison thinks none of that is a threat to Australian security, I am afraid to say, not for the first time, I disagree.

"What if Donald Trump forces Ukraine to surrender to Putin? What if Donald Trump pulls out of NATO? Donald Trump has threatened to pull out of NATO.

"Donald Trump stood up in front of an audience and he said to an unnamed European leader, if you don't spend more money on defence I'm going to encourage Putin to have a go at you. That's more or less what he said.

That, Mr Turnbull says, will "raise some very real issues" for Australia.

"We have to get used to the fact that the United States may not be aligned on the same values in quite the same way as it was 20, 30, 40 years ago and that's a reality we've got to live with," he said.

'Hamas wanted this war'

Malcolm Turnbull — who described himself as "a friend of Israel" — was also drawn into debate over the ongoing conflict in the Middle East.

He called the initial Hamas attacks on Israeli towns in October "brutal, violent, criminal, obscene" but said the Israeli bombardment in response was exactly what the terrorist group wanted.

"Hamas wanted the Israeli defence force to invade Gaza and to kill Gazan civilians," he said.

Pushed by Q+A host Patricia Karvelas on whether Israel’s response had gone too far, Mr Turnbull posed two questions.

"The question you have to ask yourself … is the price in Palestinian civilian lives now so high that the claim that the Israeli defence forces make that they're doing everything they can to protect human life no longer credible?

"And this is from an Israeli point of view, their question has to be, are they losing so much public support in the world today that they are in effect achieving what Hamas wanted to achieve?"