Prime Minister Malcolm Turnbull's message of Australian support was delivered directly to United States vice president Mike Pence in a phone conversation.
Mr Turnbull says he reassured Mr Pence bilateral ties between Australia and the United States remain strong.
The exchange comes as US president Donald Trump says the United States and its allies are safe despite his threat to rain "fire and fury" on North Korea if attacked.
He has also refused to rule out a pre-emptive strike.
Mr Turnbull has told Radio 3AW in Melbourne Australia stands by its allies.
"We would come to the aid of the United States. Now how that manifests itself will obviously depend on the circumstances and the consultations with our allies. But be under no misapprehension, in terms of defence, we are joined at the hip.* The American alliance is the absolute bedrock of our national security."
In Mr Trump's latest threat of what has become a building, tit-for-tat exchange of threats with North Korean president Kim Jong-un, he says the North should be "very, very nervous."
"If North Korea does anything in terms of even thinking about attack of anybody that we love, or we represent, or our allies, or us, they can be very, very nervous, I'll tell you what. And they should be very nervous, because things will happen to them like they never thought possible, okay? He's been pushing the world around for a long time."
When questioned further, President Trump did not back down from his threats.
(Reporter:) "Mr President, the North Koreans have said yesterday that your statement on Tuesday was nonsense -- that's the word that they used. Do you have any response to that?"
(Trump:) "Well, I don't think they mean that, and I think they ... it's the first time they've heard it like they heard it. And, frankly, the people that were questioning that statement, was it too tough ... maybe it wasn't tough enough. They've been doing this to our country for a long time, for many years, and it's about time that somebody stuck up for the people of this country and for the people of other countries. So, if anything, maybe that statement wasn't tough enough. "
Relations between the two countries have been tense for months following repeated North Korean missile tests.
In Australia, Foreign Affairs Minister Julie Bishop has described the language used over the crisis as bellicose and provocative.
And, now, two former prime ministers, Kevin Rudd and Tony Abbott, are calling for Australia to upgrade its missile-defence system.
Malcolm Turnbull says the Government is constantly reviewing its position.
"As far as a missile-defence system, the current advice from Defence to the Government is that they do not consider that there is a benefit to deploy a system such as the THAAD system -- that's Terminal High Altitude Area Defence, a bit of a mouthful -- for the defence of Australian territory. The reason for that is that THAAD's designed to provide protection for relatively small areas against short- to intermediate-range missiles. So it's deployed in Israel, it's deployed in South Korea. And it's not designed to provide protection against long-range, intercontinental ballistic missiles of the sort North Korea has recently tested."
Meanwhile, former foreign affairs minister Bob Carr has told Sky News that allies of the United States have reason to be concerned about Mr Trump.
"This confirms all our worst fears. This American president is making it up on the run. We are left with the problem of determining whether his bold statements and his tweets are to be taken as an indication of what America will do or whether we should listen to the more sober utterances of the members of his cabinet."
Mr Trump's latest comments, indeed, included another twist on his thinking about nuclear confrontation.
"I would like to de-nuke the world. I know that President (Barack) Obama said global warming is the biggest threat. I totally disagree. I say that it's a simple one: Nuclear is our greatest threat worldwide. Not even a question, not even close. So I'd like to de-nuke the world. I would like Russia and the United States and China and Pakistan and many other countries that have nuclear weapons (to) get rid of them. But until they do, we will be the most powerful nuclear nation on earth by far."
