Kevin Rudd has accused the prime minister of acting irresponsibly by pledging Australia's support for the US if North Korea attacks America.
The former prime minister took a swipe at Malcolm Turnbull's commitment to follow through on the ANZUS treaty, arguing there was no need to lock in specifics.
But one of Australia's defence ministers says Mr Turnbull was only explaining how the ANZUS treaty works and any decision to commit troops was one for cabinet.
Mr Turnbull last week confirmed Australia would come to the aid of the US should if it was attacked, insisting both nations are joined at the hip on defence.
What form that aid took would depend on the circumstances and consultation between the two nations, he said.
But Mr Rudd questioned whether conservatives in Australia had learnt anything from the Iraq war.
"John Howard gave President Bush a blank cheque on Iraq," he told ABC radio on Monday from New York.
"You never as an Australian prime minister, as an ally of the United States, give the Americans, before the event, a blank cheque."
However, Defence Industry Minister Christopher Pyne said the treaty required that Australia consult the US in the event of an attack.
The decision to commit Australia's military forces was one the national security committee of cabinet and then the full cabinet would make, and it wouldn't be made lightly.
"We aren't giving anyone a blank cheque," Mr Pyne told ABC radio.
"Kevin Rudd, I think, should recognise this is a serious matter and not one for political point scoring."
He believed the situation was a long way from the possibility of war.
"We are at the moment using all diplomatic efforts to ensure that we never get to that point because the results of any war, especially if initiated by North Korea, are too terrible to think about," he said.
Mr Rudd and his successor Tony Abbott have called for Australia to obtain a missile defence shield, similar to America's THAAD system.
Mr Pyne dismissed that as "patently absurd".
It would cost tens of billions of dollars, take years to build and would be useless against the intercontinental, long-range missiles that were the only likely possibility of hitting Australia, he said.
Crossbench senator Nick Xenophon said the ANZUS treaty was not about giving the US a blank cheque.
"It's about being partners in a very difficult situation," he told reporters in Canberra.
Australia's war powers needed to be amended to give parliament greater scrutiny of overseas deployments.
"We can't sleepwalk our way into a conflict," Senator Xenophon said.
"That would have profound and disastrous implications for the Australian people."
