Australia will only receive used nuclear-powered submarines from the United States as part of an agreement to "streamline" the AUKUS deal, with the move branded on Sunday as a "cost-effective" measure by Defence Minister Richard Marles.

The two nations — together with the third partner in their security pact, Britain — met at Singapore's Shangri-La Dialogue, which brings together top defence officials and experts from about 45 countries.

Under the 2021 AUKUS deal, Australia is expected to receive at least three so-called "Virginia-class" nuclear-powered submarines from the United States within 15 years.

Australia had been expecting to receive two used submarines and one new one, but the countries announced Saturday (May 31, 2026) that all three will now be in-service vessels from the U.S. Navy stock.

When asked why Canberra was now receiving only used equipment, Mr. Marles, who is also Deputy Prime Minister, told reporters on Sunday (May 30) it would be more cost-effective.

"In the context of a very complicated endeavour, we need to place a premium on simplicity," said Mr. Marles, who added that the submarines will also be the same model.

"I cannot overstate the significance of that, both in terms of the submariners who are operating them, but also the people who are working on them to sustain those submarines," Mr. Marles said.

"It is definitely cost-effective. And to be clear, this is a very expensive programme, and so we are trying to find every cost-effective option as we walk down this path."

In a joint statement on Saturday (May 30), Mr. Marles, U.S. Minister for Defence Pete Hegseth, and the U.K. Secretary for Defence John Healey confirmed the tweak to the submarine agreement.

"The Deputy Prime Minister and secretaries welcomed the proposed approach to streamline Australia's acquisition of Virginia-class submarines (VCS), simplifying supply chain management, operational and maintenance requirements, and maximising cost efficiencies," the statement said.

"This approach would enable Australia to acquire three in-service VCS instead of a mixture of new and in-service VCS variants." The U.S. Navy has 24 Virginia-class vessels, but American shipyards are struggling to meet production targets set at two new boats each year.

In the United States, critics have questioned why Washington would sell nuclear-powered submarines to Australia without stocking its own military first.

The AUKUS submarine programme lies at the heart of Australia's defence strategy and could cost up to $235 billion over 30 years, according to government forecasts.