Government says it preferred second-hand submarines

A change not to deliver a new Virginia-class submarine under the AUKUS pact has faced scrutiny. (Supplied: US Department of Defense)

Changes to the AUKUS deal mirror Australia's initial preference of receiving a second-hand submarine, Senate estimates has revealed.

Labor backbencher Ed Husic has called into question the future of the AUKUS pact and argues Labor needs to renegotiate the deal.

Mr Husic will not be drawn on whether others in the Labor caucus feel the same way, but says there is clear "disquiet" within Labor's rank and file on the issue.

It was always Australia's preference to get three second-hand submarines, a senior defence official has revealed in Senate estimates, as the government faces pressure to renegotiate the AUKUS pact.

The revelation casts a new light on changes to the agreement over the weekend, under which Australia will buy three used Virginia-class submarines rather than two used and one new.

The US, UK and Australia have announced they will "streamline" the purchase, with Australia no longer buying any new submarines, and acquiring three second-hand submarines.

Defence Minister Richard Marles met with US War Secretary Pete Hegseth in Singapore on Saturday, where the pair reaffirmed the AUKUS pact but announced the change to the delivery plan.

Mr Marles has argued that acquiring three used submarines in the 2030s would "streamline" an "incredibly complicated" deal.

Secretary of the Defence Department Meghan Quinn confirmed it was a "joint idea" to modify the agreement, three years after the nuclear-powered submarine pathway was announced.

"Australia's position is that we would have always had a preference for three in service," she told Senate estimates on Tuesday.

Liberal senator James Paterson queried why the United States had "imposed" a new submarine on Australia.

"Because this is a joint exercise over many decades working collaboratively with an alliance partner to deliver a capability, which is significant and is very important for Australia's national defence," Ms Quinn said.

She said ongoing discussions over time had delivered the new arrangement, which would be "simpler" and "lower cost".

"Just a note that these in-service Virginia-class submarines would be in the early stage of their operation and would be available to Australia for decades after receipt of them," she added.

Labor MP Ed Husic says Australia needs to look at renegotiating the AUKUS pact with the United States and the United Kingdom and start contemplating alternative options following the changes.

Ed Husic says it needs to be questio