US-Australia talks on AUKUS changes have been underway for 18 months

USS Texas is one of the US Navy's Virginia class submarines. (Supplied: US Department of Defense)

Talks on changes to the AUKUS pact began around 18 months ago, according to Australian Submarine Agency officials.

The changes announced over the weekend will see Australia buy three second-hand submarines, rather than two second-hand and one new.

Officials told the hearings the submarines Australia buys will be roughly six years old, and have a life span of 30–33 years.

Australian officials began talking with their US counterparts about changing the AUKUS pact around a year and a half ago, a Senate estimates hearing has heard.

Discussions began early last year, culminating in an announcement last weekend that Australia would buy three second-hand Virginia-class submarines in the 2030s, rather than two second-hand and one new.

The change is the most significant shift to the AUKUS "optimal pathway" since it was announced in 2023.

The government has this week argued that buying three "in-service" submarines has always been the preferred option, given the simplicity of operating three near-identical submarines.

Officials from the Australian Submarine Agency, which is tasked with overseeing Australia's acquisition of nuclear-powered submarines, were questioned at Senate estimates on details of the changes.

Vice Admiral Jonathan Mead heads up the Australian Submarine Agency. (Supplied)

Vice-Admiral Jonathan Mead was asked when "intense discussions" with the United States about the prospect of changing the plans began.

Australia will only buy second-hand Virginia nuclear-powered submarines from the US during the AUKUS stopgap phase, with some analysts criticising the move and others believing it is a positive change of plan.

He said Australia had agreed to the best deal available in 2023, but there had been ongoing conversations about shifting to three "in-service" submarines.

Vice-Admiral Mead said there were constraints on the US in 2023 that meant they could not agree to selling three in-service submarines, but that had changed over time.

"What was available to us then was the optimal pathway," he said.

"We have been working with the US now for about the last 12 months or more on seeing if there was scope — as their industrial base matured … as they began to climb their production rates — was there scope to turn production into an 'in-service'.

"That has been an ongoing conversation with the US, which we saw was agreed to."

The government has also argued the change will see budget savings, as it is cheaper to both buy and maintain an in-service