The Trump administration has escalated its rhetoric against North Korea, after the rogue regime threatened to attack the US territory of Guam in the Pacific.
"America stands by its allies, including Australia of course, and we stand by the United States," Mr Turnbull told 3AW.
"So be very, very clear on that. If there's an attack on the US, the ANZUS Treaty would be invoked and Australia would come to the aid of the United States, as America would come to our aid if we were attacked."
New Zealand was an original partner of the three-way ANZUS Treaty when it was signed in 1951, but was suspended in 1985 in the rift over anti-nuclear laws, and is no longer a formal ally.
New Zealand and the United States have a defence co-operation agreement - the 2012 Washington Declaration - but there is no obligation to come to each other's aid.
Tensions have been rising on the Korean peninsula after North Korea carried out two intercontinental ballistic missile tests last month.
Mr Trump has warned he would not allow Pyongyang to develop nuclear weapons capable of hitting the US.
Mr Turnbull discussed North Korea's behaviour with US Vice-President Mike Pence overnight, calling it "the most dangerous flashpoint in the world today".
He would not say if Mr Pence sought any specific assurances of military aid from Australia.
But the Prime Minister said the terms of the Australia-US alliance were clear.
"But be under no misapprehension, in terms of defence, we are joined at the hip," he said.
"The American alliance is the absolute bedrock of our national security. If there is an attack on the US ... we would come to their aid.
"Now, how that manifests itself obviously will depend on the circumstances and the consultations with our allies."
Mr Turnbull said Australia and the US both still believed tough new sanctions on North Korea could force the regime to abandon its nuclear weapons program.
"[The Vice-President's] view and the view of the administration is that the way to resolve the situation with North Korea … is through these economic sanctions. That's the preferred way to deal with it," he said.
"But of course if North Korea decides to carry out some of its violent threats then obviously terrible consequences will follow, and there's no point ducking that inevitable consequence."
On Tuesday Mr Trump warned North Korea it would be "met with fire and fury like the world has never seen" if it continued to threaten the US.
The US President has not backed away from those remarks, today saying his language could have been even tougher.
"Maybe it wasn't tough enough. They've been doing this to our country for a long time, for many years," he said.
Photo: AFP US President Donald Trump and Australian PM Malcolm Turnbull.