The Chief of Navy says he expects Australia's two largest and most expensive warships will be back in service by October this year.

Australia's two Landing Helicopter Docks (LHDs), HMAS Canberra and HMAS Adelaide, have been docked in Sydney since March after problems were discovered with their propulsion systems.

Facing a Senate Estimates hearing, Chief of Navy Vice Admiral Tim Barrett confirmed HMAS Adelaide will remain in dry dock at Garden Island where engineers are closely examining her propulsion pods.

"If there's anything that we discover from her that we then need to apply with [HMAS] Canberra [that] will be done in that docking in the third quarter of this year," he said.

"The expectation [is] that both ships will be able to be back in service by the end of the fourth quarter of this year."

During extensive questioning from Labor senator Kim Carr, the Navy Chief acknowledged Australia's newest warships will have spent more time out of service this year than on operations.

"They will have been alongside for more time than they will have been at sea, Senator, that's correct," Vice Admiral Barrett said.

Earlier this month, the Navy confirmed HMAS Adelaide would miss highly-anticipated war games with the United States at the end of June, and said it was too early to say whether HMAS Canberra would be able to participate in Exercise Talisman Sabre — even in a reduced capacity.

Sorry, this video has expired HMAS Adelaide enters dry dock in Sydney for urgent repairs

'Too early' to say how much repair bill will be

Defence's head of maritime systems Rear Admiral Adam Grunsell said it was also too early to say how much it would cost to repair its two largest warships, and some of the eventual bill may be covered by warranty.

"It will aggregated at the end of the activity or towards the end of activity," he said.

"I can't give you the exact cost at this stage."

Rear Admiral Grunsell has previously said a design flaw could not be ruled out as a reason for the propulsion problems — although senior Navy figures say the latest testing points to failures with seals used inside the high-tech azimuth pods on the LHDs.

HMAS Canberra was commissioned into service in 2014, while her sister ship HMAS Adelaide was commissioned 18 months ago, with both LHDs costing a total of about $3 billion.