Future submarine costs, due dates blow out

Australia's future submarines program has hit yet another delay and the expensive new fleet will cost even more than expected.

Defence will begin signing off on the final design of the submarines nearly a year after this year's June deadline.

The expected cost has blown out by nearly another $10 billion on top of the already expanded $80 billion price tag.

Defence and naval heads overseeing the build pointed to foreign exchange rates and inflation for the now $89.7 billion cost to taxpayers.

The defence department's deputy secretary in charge of national shipbuilding, Tony Dalton, told a parliamentary inquiry on Wednesday the project was exposed to the euro, US dollar and British pound.

"It could go higher or it could go lower depending on foreign exchange rate variations," Mr Dalton said.

"I don't have a crystal ball."

The design of the new fleet has so far cost taxpayers nearly $1.2 billion as of January this year, the committee heard.

Its new critical design review, slated to begin next month, will now begin in May 2023 with construction to begin in 2024.

The first of the submarines to replace Australia's ageing Collins-class fleet is expected in 2034, with the last due by 2054.