Australia and Papua New Guinea have inked a landmark mutual defence pact, while playing down a lack of unity behind the deal in Port Moresby.

Prime Minister Anthony Albanese and PNG counterpart James Marape signed the Pukpuk agreement , named after the local word for crocodile, at Parliament House in Canberra on Monday.

The treaty means Papua New Guinea joins the United States and New Zealand as the third defence ally of Australia, and the newest addition in 70 years.

The signing followed a botched attempt in Port Moresby in September, when the trans-Torres treaty was hoped to be confirmed alongside PNG celebrating its 50th anniversary of independence.

The treaty couldn't get the green light from Mr Marape's cabinet in time, delaying the announcement and moving it to Australia.

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