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 neighbors, 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 2022, Beijing signed a security deal with the Solomon Islands which has seen Chinese police officers embedded across the country, with another policing agreement forged in 2023.
In response, Canberra last December struck a deal to invest A$190m ($126m; £93m) into the Solomon Islands police force and set up a police training center, with a similar agreement in place with Tuvalu.
PNG Prime Minister James Marape, who signed this latest agreement with his Australian counterpart Anthony Albanese on Monday, stressed the deal was not born out of geopolitics.
PNG has been transparent with China, Marape said while in Canberra.
We have told them that Australia is our security partner of choice and they understand our alliances here... Other aspects of our relations have never been compromised, he said.
Albanese said the two countries' alliance is built on generations of mutual trust, and demonstrates our commitment to ensuring the Pacific remains peaceful, stable and prosperous.
The Pukpuk Treaty, named after the word for crocodile in PNG pidgin, notes that an armed attack on either country would be dangerous to the other's peace and security, so both should act to meet the common danger.
The deal also covered greater collaboration around cyberspace and electromagnetic warfare. Earlier, the PNG Defence Minister Billy Joseph had told the ABC that the deal would mean Australian and PNG forces would be totally integrated.
The deal will help modernize PNG's army, according to expert Anna Powles, bringing both material and morale boosts.
However, critics express concerns regarding how it aligns with PNG's longstanding foreign policy of maintaining neutrality.
