Australia will gain access to Papua New Guinea's (PNG) military facilities and troops under a key deal that will see the nations come to each other's aid if either is attacked.
Both governments say the deal was born from a yearslong alliance between the two Pacific neighbours, but experts say it is aimed at countering China's growing influence in the region.
The deal ensures China will not have the same access to infrastructure in PNG as it does in other Pacific Islands, said Oliver Nobetau, project director of the Lowy Institute's Australia-PNG network.
It will allow as many as 10,000 Papua New Guineans to serve in Australia's military, and give them the option to become Australian citizens.
With nearly 12 million people, PNG is the largest and most populous South Pacific nation.
China has already significantly shored up trade with Pacific Island nations in recent years, and is now trying to establish diplomatic and security beachheads across the region.
Australia and its Western allies, including the United States, have been attempting to counter these efforts.
In response, Canberra last December struck a deal to invest A$190m ($126m) into the Solomon Islands police force and set up a police training centre, with a similar agreement in place with Tuvalu.
The Pukpuk Treaty, noted to signify strength and readiness, states that an armed attack on either country would be dangerous to the other's peace and security, necessitating mutual action against common dangers.
Furthermore, the agreement includes annual joint military exercises and cooperative efforts in cyberspace and electromagnetic warfare, signaling a concerted front against external threats.
While the pact marks a historic strengthening of ties, it has not been without controversy in PNG, with concerns raised over the potential implications on the country's long-standing foreign policy of neutrality.
