Impact of Australia only buying second-hand US submarines under AUKUS

Australia will be buying three used Virginia-class submarines as part of the stopgap phase of AUKUS. (Reuters: Brian Snyder)

Australia is set to only buy second-hand US nuclear submarines under the stopgap phase of AUKUS.

The three Virginia-class submarines are expected to be the Block IV model, which started entering service in 2020.

Defence experts say there are benefits in purchasing three of the same used submarines, rather than adding a new one.

Under the initial AUKUS agreement, Australia was set to receive two used Virginia-class nuclear-powered submarines from the United States, plus one new model as early as 2032.

But the plan has changed and now all three submarines will be second-hand, a decision Defence Minister Richard Marles said would save "significant" amounts of money and simplify fleet training and maintenance.

The Virginia-class vessels are part of a stopgap phase to boost the Royal Australian Navy's capabilities until the first AUKUS-class nuclear-powered submarines start arriving from about 2042.

Some analysts have been critical of the pivot, questioning whether relying on used submarines would reduce Australia's naval capabilities.

But others say the older fleet could work in Australia's favour.

Under the original plan, the US was selling Australia two "used but refurbished" Block IV Virginia-class submarines.

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.

The third was intended to be a newer Virginia-class model straight from the production line.

But now the purchase would be streamlined and all three would "be of the same type", Mr Marles said.

Richard Marles (centre) announced the AUKUS changes alongside John Healey (right) and Pete Hegseth in Singapore. (Reuters: Edgar Su)

Marcus Hellyer, head of research at defence think tank Strategic Analysis Australia, said there had been a lot of "hand wringing" about acquiring older vessels.

But the Block IV submarines only started entering service between 2020 and 2026, with the remainder of the fleet set to be finished by next year.

"These are the boats the US is currently finishing, so they're not old in a sense of old," Dr Hellyer told the ABC.

Marcus Hellyer does not believe Australia's submarine capabilities will be compromised without a new Virginia-class vessel. (ABC News: Ian Cutmore)

The average life span of the Block IV submarines is about 33 years.

The first one is expected to arrive in Australia in 2032, with the additional vessels be