One of the Royal Australian Navy's largest warships, HMAS Adelaide, has been dry-docked as naval engineers scramble to fix engine problems with the $1.5-billion vessel.

It is still unknown how long it will take to repair the Landing Helicopter Dock (LHD), which was only commissioned into service 18 months ago.

HMAS Adelaide's sister ship HMAS Canberra also remains out of action, berthed at Sydney's Garden Island Naval base.

"You don't expect it when it's a few years old. This is capability that we should be confident that we got the specifications right and it should be operational," said David Smith from Professionals Australia, a union representing scientists and engineers in Defence.

"It's incredibly significant given the billions of dollars we're investing in naval capability."

In March, the ABC revealed HMAS Canberra and HMAS Adelaide had been sent to Garden Island after problems were identified with their propulsion systems.

Some Defence insiders fear the engine issues will not be resolved before next month's Talisman Sabre military exercises. ( ABC News: Billy Cooper )

HMAS Adelaide was moved into dry dock at Garden Island early on Wednesday morning but Defence is giving few details on the progress of the investigation and repair work.

"During first-of-class flight trials, Defence identified an emergent issue with the propulsion systems of HMAS Canberra. HMAS Adelaide was also inspected and there are indications of a similar issue," the Defence Department said in a statement.

"Defence has adopted a deliberate and disciplined approach to resolve this problem early in the ships' life."

Defence Minister Marise Payne has also expressed frustration at the time taken to repair mechanical problems with the Navy's two largest ships.

"This is the period of time in which, in Defence acquisition and implementation terms, we've identified these sorts of issues and we address them," she said.

"That's not to say that it's not frustrating that it has been an issue that's under examination for some time."

Some Defence insiders fear the engine issues on both ships will not be resolved before next month's Talisman Sabre military exercises with the United States, but the department and Minister are refusing to speculate.

"Whatever time that takes is frankly the time that it will take. I don't intend to ask them to rush it, I don't intend them to work to an artificial or self imposed deadline from anyone else," Ms Payne said.

Labor MP David Feeney is demanding the Government reveal publicly exactly what is wrong with the Royal Australian Navy's two LHDs.

"This Government I think now needs to step up to the plate and explain precisely what the circumstances are," he said.

"Moving these ships in the dead of night might be tricky but it's not actually telling the Australian people what we need to know — what is the status of these ships," he said.