Chief of Navy Michael Noonan is assessing how many of the six ageing vessels will need major work to keep them in service.
"We are yet to fully determine how many of the boats we will upgrade," Vice Admiral Noonan told a Senate estimates hearing in Canberra on Wednesday.
"We're expecting that we will upgrade at least five, and the work around determining the scope of the upgrade has begun but has not yet been fully decided."
Defence had planned to retire the Collins Class submarines from 2026, but has since decided to prolong their lives until the new fleet arrives.
The first of the French-built vessels is expected to be delivered in 2032 and enter service about three years later.
Australia has signed a $50 billion contract with the French state-owned company Naval Group to deliver 12 submarines.