Defence is still trying to figure out why a $100 million navy vessel bought last year from the United Kingdom and given a clean bill of mechanical health broke down.
Defence Materiel Organisation chief executive Warren King has told a senate budget estimates hearing one of two main propulsion transformers aboard the amphibious landing ship HMAS Choules had failed.
The catastrophe occurred in June when Choules sailed north from Sydney to participate in Exercise Hamel and had to return on reduced power.
Later, a hole was cut in the ship to remove the dodgy transformer, which weighs 6.5 tonnes.
An inspection revealed the insulation had failed prematurely, leading to a major short circuit, but fortunately no fire.
Further inspections of four other power transformers also revealed premature wear.
Mr King told the hearing in Canberra the Defence Science and Technology and manufacture Siemens were trying to find out why the propulsion transformer failed.
Some of the causes could be over-heating, although temperature sensors did not indicate this, or mechanical stress.
'We have certainly run into a serious problem with this transformer and we are not yet at the end of the path of knowing what's caused the problem,' Mr King said.
Navy chief Vice Admiral Ray Griggs said new transformers had been ordered, costing $1 million each, but it hadn't yet been decided whether all would be replaced.
Repairing Choules could cost up to $10 million and it won't be able to return to service until January at the earliest or April at the latest.
The UK has reported no similar experience, although it's about to inspect its vessels for signs of premature transformer wear.
The 16,000 tonne Choules was built in the UK, launched in 2003 and served in Britain's Royal Fleet Auxiliary from 2006 until 2011, when she was declared surplus and sold to Australia.
Unlike vessels with diesel engines that directly drive the propellers, Choules' main engines run generators providing power to electric motor propulsion pods through transformers.