The Albanese government says it will be changing its sustainment approach to the submarines, meaning not all six boats will have key systems such as engines and motors replaced.
Addressing the Lowy Institute on Tuesday, Defence Minister Richard Marles said the $11 billion program would be "conditions-based".
"This will see a pivot in our approach that reduces risk, upgrades capability and maximises availability for the navy," he said.
"We will prioritise sustainment and accelerate upgrades for the fleet's youngest submarines.
"The program will reduce engineering risk by sustaining existing systems where appropriate while continuing to upgrade critical capabilities, including weapons and combat systems."
The government claims the change won't impact the plan to give each submarine an extra 10 years of life.
But former naval officer Jennifer Parker said it was an "unlikely aspiration" the submarines would last into the late 2030s or early 2040s.
"Until we're replacing them with the Virginia-class, the submarines were always going to be limped through ... the fact is they should have been replaced or started to be replaced by now," she told AAP.
"When it comes to the Collins-class, we should be spending as little as possible to keep them safe and at sea to link them through until we get the Virginia-class.
"Spending too much money on upgrading them with new systems would be a waste of money."
Australia has been promised three Virginia-class submarines in the early 2030s under the AUKUS partnership with the US and UK.
HMAS Farncomb, the second of the Collins-class submarines to enter service in the Australian Navy, will undergo its extension service within weeks and will take longer than two years.
The other submarines will be individually assessed.
The navy has operated the fleet which entered service from the mid-1990s, but the boats have recently faced corrosion issues and maintenance delays.
Opposition defence spokesman James Paterson said Mr Marles had "used the cover of a remarkably partisan and defensive speech" to admit a major de-scoping of the Collins-class life of type extension.
"As is often the case, he's failed to provide the details transparently to the public," he said.
"Which aspects of upgrades previously planned are now being abandoned? And how much is the Albanese government cutting from the program?"
The Collins-class boats were listed a "product of concern" in late 2024, as only one of the six submarines were fully operational.
This was done to increase ministerial oversight and management of the submarines.
Defence Industry Minister Pat Conroy said the life of type extension was a critical capability decision.
