Australia's new submarine fleet may end up with conventional propellers as well as air independent propulsion, which helps increase underwater endurance.
The French shipbuilder commissioned to design Australia's new fleet of submarines has bedded down the size and shape of the vessels.
Naval Group executive director of the Australian submarine program Jean-Michel Billig says the decision represents an important milestone in the pre-design phase of the $50 billion program.
"We know what shall be the length, what shall be the diameter of the submarine," he told reporters at the Pacific 17 maritime showcase in Sydney on Wednesday.
But he declined to provide the specific details because it's sensitive information.
The next priority is recruiting and pre-qualifying Australian supply-chain companies for the project and Naval Group has been running a series of roadshows across the country.
The vessels may end up with conventional propellers as well as air independent propulsion, which helps to increase underwater endurance, Mr Billig said.
An independent report has raised concerns Australia could have a capability gap between its new fleet coming into service and the existing six Collins Class submarines being withdrawn by 2036 because of possible project delays.
Mr Billig dismissed fears about engineering difficulties.
"We are not converting a nuclear-powered submarine into a conventional submarine, we are starting from scratch," he said.
Cabinet minister Christopher Pyne vowed the project would remain on schedule "as long as I'm the minister for defence industry".
"What you're asking me to do is guarantee something in five, six, 10, 12 years, I might not even be alive," he joked.
Mr Pyne expects there to be at least a 60 per cent local build.
Asked about speculation of a possible merger between Naval Group, French defence firm Thales and Italian shipbuilder Fincantieri and the implications for Australia, Mr Billig said: "It has no impact at all".
"We won't have Fincantieri in any way intervening."
Fincantieri is bidding for the contract to build Australia's nine new frigates, up against British BAE Systems and Spanish Navantia firm.
The winner will be announced in 2018.