Australia will have to break out the chequebook to hold up its end of a historic defence alliance with Papua New Guinea, analysts say.

Prime Minister Anthony Albanese and his PNG counterpart James Marape are expected to ink a long-awaited mutual defence treaty as early as Monday after attending Sunday's NRL grand final.

The Pukpuk agreement, named after the local word for crocodile, will require each nation to come to the aid of the other if under military attack.

It will also create a pathway for citizens to serve in the other country's defence force.

The expected treaty will be Australia's first formal alliance since the ANZUS treaty was signed in 1951.

"It's only the second alliance Australia will have and our third treaty ally," ANU National Security College senior policy advisor

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