Australia expects to make significant cost savings by buying a third secondhand American nuclear submarine for the Aukus program, instead of at least one brand new model.The defence minister, Richard Marles, on Sunday described the decision not to buy any new Virginia-class boats as placing “a premium on simplicity”.But Marles conceded there would be no “fundamental” shift in the cost of the multi-decade deal, estimated to be worth at least $370bn.The government had expected to buy one new and two secondhand Virginia-class submarines. But Marles said on Sunday the decision to go with three “in-service” submarines would make training and operations easier and cheaper for Australian crews onboard from the early 2030s.“We are trying to find every cost-effective option as we walk down this path,” Marles said on the sidelines of the Shangri-La Dialogue in Singapore on Sunday.Sign up for the Breaking News Australia email “This is a very useful financial contribution to the cost of the overall program, but actually at the heart of this is chasing simplicity.”Marles played down the cost of Aukus as 0.15% of GDP over the lifetime of the deal.After meetings with his US and UK counterparts, Pete Hegseth and John Healey, Marles said servicing and training efforts would be streamlined because Australian crews would not operate two different American-made submarines before the bespoke SSN Aukus model comes online in 2042.The first Virginia-class from the United States is due to arrive in Australia in 2032, with another arriving every four years, before the Australian-built model is ready for operations.“Chasing simplicity is at the heart of why we have pursued this,” he said, stressing Australia was confident the Aukus timeline was on achievable.Until the Aukus fleet arrives, Australia is extending use of 30-year-old Collins-class submarines, keeping the six Adelaide-built boats in the water for an extra 10 years.The US government is attempting to double the rate of production of the Virginia-class to boost its own capacity and facilitate Aukus.American shipyards currently produce between 1.1 and 1.2 Virginia-class submarines each year, well below the target yearly rate of 2.33 needed for the deal to go ahead as planned.“We have a sense of confidence about the production rates getting to where they need to get to,” Marles said.“There’s about 200 Australian tradespeople at Pearl Harbor, working on getting Virginia-class submarines out to sea for the US Navy. That’s a part of increasing the number of sea days for the US Navy with their Virginia-class submarines.”But there is growing disquiet within Labor over Aukus.Marcus Strom, the national convener of Labor Against War, said Marles was compelled to accept “dodgy Pete Hegseth’s second-hand subs as ‘significant savings’”.Strom said that from the US perspective, whether Australia ever got Virginia-class submarines was unimportant.