Defence has denied it has a capability gap, despite two of the navy's largest ships being prevented from helping in the Cyclone Debbie recovery.
HMAS Canberra is back in Sydney's Garden Island Naval Base as divers assess a propulsion problem.
There were indications a couple of weeks ago something was wrong, but it was only once the ship returned from Queensland that investigations could begin.
"It may need to be dry-docked," Chief of Navy Tim Barrett admitted to a Senate committee in Canberra on Thursday.
HMAS Adelaide is also being looked at, but officials rejected claims rust was a factor.
"I was on Adelaide for four hours yesterday with some of the smartest engineers from all those companies and within the navy and public service and I didn't hear the word 'rust' mentioned once," Deputy Secretary Kim Gillis said.
Vice-Admiral Barrett said the navy was already looking into its supplies of spare parts and whether others need to be brought from Spain, Germany or other parts of the world.
"The investigation is not being held up by not having spare parts," he said.
He couldn't say when the ships would be operational again, pending the investigation.
But it was his desire for the ships to participate in the upcoming biennial combined Australian and United States training activity, Exercise Talisman Sabre.
Asked by Labor senator Alex Gallacher if the force had a capability gap, Vice Defence Force Chief Ray Griggs said HMAS Choules was the on-call humanitarian and response vessel.
"It was deployed several days before Cyclone Debbie struck and it is on route to the Whitsundays as we speak," he said.
The vice-admiral was appearing for Mark Binskin, who was touring cyclone affected parts of North Queensland with Prime Minister Malcolm Turnbull and Labor leader Bill Shorten.
