The federal government is now scheduled to buy three Virginia-class submarines already in service, instead of a mix of new and used US vessels.
Defence Minister Richard Marles said the trilateral agreement was "incredibly complicated" and any move to simplify it would be beneficial.
"In the context of a very complicated endeavour, we need to place a premium on simplicity ... what we will have here is a much simpler pathway," he told reporters on Sunday from a major summit in Singapore.
"It will mean the Virginia-class submarines that we are acquiring will all be of the same type."
The change would be of significant benefit for the submariners operating the vessels and the workers maintaining them, Mr Marles said.
While the decision to buy second-hand would not materially change Australia's defence budget, the minister said it was a more cost-effective option.
"This is a very useful financial contribution to the cost of the overall program," he said.
Under the original AUKUS deal with the US and the UK, Australia would receive a mix of new and used American submarines from the early 2030s.
The nation would eventually build its own vessels locally using US nuclear-propulsion technology and some parts made in Britain.
But the US has struggled to make enough Virginia-class submarines for its own use, leading some American lawmakers to question how it could also build enough of the boats to meet Australia's needs.
Australia faces a critical window to receive the submarines before its existing, 1980s-designed Collins-class vessels are retired from service.
The shift in the nuclear-powered submarine plan follows the announcement of trilateral plans to build unmanned undersea vehicles as part of the second "pillar' of the AUKUS pact, covering the sharing of defence technologies between the allies.
The project will help secure undersea cables and pipelines from sabotage, US Secretary of War Peter Hegseth said on Saturday while standing alongside Mr Marles and their UK counterpart.
"The signature project will deliver a suite of highly adaptable multi-mission UUV payloads designed to support undersea operations and maintain our collective advantage in the maritime domain," Mr Hegseth told reporters.
