Malcolm Turnbull intends talking up international collaboration in the battle against terrorism during meetings in the US.
The prime minister arrived in New York on Saturday ahead of the 71st session of the United Nations General Assembly, making his first stop the 9/11 memorial and museum with wife Lucy.
Later reflecting on the attacks 15 years ago which were many months in the making, he warned the nature of terrorism continues to "mutate".
"We have to ward ourselves against attacks like (9/11) in the future," Mr Turnbull told reporters.
But there is also the threat of lone wolves, who are rapidly radicalised.
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"That's why we have to be relentless in our defence of our freedom," he said.
Countries have to be ever vigilant in the face of the evolving threat.
Mr Turnbull described terrorism as a technique and said any incident is examined with great care so lessons can be learnt.
Later in the week he will meet with Defence Secretary Ash Carter and senior national security and intelligence officials in Washington DC on global and regional security issues.
