The Chief of Navy has signalled Australia's entire fleet of ageing Collins Class submarines might need upgrading before its $50 billion French-built replacements are ready.

Key points: Australia to replace its ageing submarines with 12 French-built vessels costing $50 billion

Australia to replace its ageing submarines with 12 French-built vessels costing $50 billion Chief of Navy tells the Senate the Collins Class fleet might need upgrading before they arrive

Chief of Navy tells the Senate the Collins Class fleet might need upgrading before they arrive Collins Class submarines will be retired when the French replacements arrive

The first of the new French-designed submarines is not due to be in service until the mid-2030s.

Navy Chief Vice Admiral Mike Noonan said Defence was assessing how many Collins Class submarines would need a "life of type" extension to keep them in the water until the replacement boats were ready.

"We are yet to fully determine how many of the boats we will upgrade," he told Senate Estimates.

"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."

Australia's six Collins Class submarines were commissioned between 1996 and 2003.

Defence originally planned to retire the submarines around 2026.

But in 2016 Defence officials confirmed the submarines' service life would be extended until replacements were delivered.

Under questioning from Labor senator Penny Wong, the Chief of Navy insisted no decision on full upgrades had been made yet.

"Government has not yet made a determination as to how many Collins Class will be upgraded," he said.

"There will be a number that need to be upgraded and that number could be between three and six, depending on the nature of the capabilities that are available to put into that program as we go through the life of type extension."

Sorry, this video has expired Laura Tingle speaks to France's Armed Forces Minister Florence Parly

The French state-owned company Naval Group will deliver Australia 12 submarines.

Prime Minister Scott Morrison and the French Defence Minister Florence Parly last week signed a long awaited "strategic partnering agreement" to underpin the $50 billion Future Submarine project.

That signing came after months of tense negotiations between the two countries.

The ABC last year revealed the Federal Government had grown so frustrated with Naval Group that Defence Minister Christopher Pyne refused to meet top officials who were visiting Australia.

No agreement on Australian contribution to subs build

Senate Estimates has also heard the submarine replacement project does not include a set percentage for the amount of Australian content that would be included.

Mr Pyne last year declared the percentage of "Australian content" in the submarine deal would be well above 60 per cent.

But Defence's submarines general manager Stephen Johnson told the Senate it was too early to set a percentage.

"It's a sequential event so it's a flawed strategy to set a per cent before you have enough information," he said.

Mr Johnson said a percentage had never been put on the table in discussions with the Naval Group.

When asked by Senator Wong why Mr Pyne would have publicly announced a figure, Mr Johnson told the committee "I don't have a way of answering that".