The Navy could be left without a submarine fleet for up to 20 years because of a "wildly ambitious" schedule to replace the ageing Collins Class fleet, an independent report has found.

Key points: First new submarine to come into service same year last Collins Class to be decommissioned

First new submarine to come into service same year last Collins Class to be decommissioned Replacement fleet delay highly likely because of complex design, report's author says

Replacement fleet delay highly likely because of complex design, report's author says Without boats, ability to defend Australia affected, crew skills would deteriorate, report says

The report found the multi-billion dollar project to replace the submarines is likely to be three times more expensive than comparative builds for other countries' defence forces.

The first new submarine is due to come into service in 2033, the same year the last of the Collins Class boats is due for decommissioning.

But the group of former public servants and defence analysts who authored the report say with regional tensions on the rise the risk could not come at a worse time.

ANU strategic studies professor Hugh White, who coordinated the report, said delays of longer than normal were highly likely because of the highly complex design of the replacement fleet.

"We are here to sound the alarm and to encourage a rethink," Professor White told the National Press Club in Canberra.

"Submarines are unusually difficult to get right and, as we argue in the report, the submarine project we have underway at the moment is in unusually deep trouble."

Professor White said while delays of five or 10 years were the norm for submarine projects around the world, the Government's delay would have massive knock-on effects for the Navy's ability to defend Australia.

Crews and captains' skills would deteriorate with no boats to train on, and new crews would need to be trained up as new boats came into service.

The report suggests buying six off-the-shelf submarines from France to help fill the gap between when the Collins fleet is decommissioned and the new fleet is ready.

"We could end up with a decade or two gap in our operational submarine capability at a time of unusual strategic uncertainty in the Asia-Pacific," he said.

"And that's if everything goes according to plan, and that's wildly optimistic."

Submarine costs 'very sobering thought'

French company DCNS was last year awarded the contract for 12 new submarines to replace Australia's ageing Collins Class submarine fleet, estimated to be worth $46 billion in 2016 prices.

"That makes it on the calculations in the report up to three times the cost of other conventionally powered submarines, and that's a very sobering thought indeed," Professor White said.

Professor White's report is also critical of the decision to use existing battery technology rather than quieter lithium ion batteries.

"These are difficult technologies, but by turning our backs on them we're guaranteeing we're going to have a submarine that's not actually going to perform all that well," he said.

As part of the submarine replacement program, the French Suffren — or Barracuda — will be modified to replace its nuclear propulsion with conventional diesel electric power.

"The Suffren itself is not in service and has a long and troubled history," Professor White said.

"There are now questions about whether our submarine design will be based on the Suffren or we'll be starting from scratch."