Kevin Rudd has accused the prime minister of acting irresponsibly by pledging to back 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, saying he did not need to lock in specifics.
Mr Turnbull last week confirmed Australia would come to the aid of the US, insisting both nations are joined at the hip on defence.
But Mr Rudd questioned whether conservatives in Australia had learnt anything from the Iraq experience.
"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."
Mr Rudd said there were many scenarios which could play out on the Korean peninsula.
"But for an Australian prime minister to say we automatically would become militarily involved in the event of a North Korean attack, frankly, I think, is irresponsible in terms of our core national security interests."