There are concerns that work to extend the life of Collins class submarines could be delayed due to the complex nature of the refurbishments.
The defence department was "acutely aware of the importance of getting this right", given the Collins class was the stopgap until the nuclear submarines arrived, navy chief Vice Admiral Mark Hammond said.
The life of type extension for each boat is expected to take two years, with the first to go in for work in 2026.
The fleet needs to be extended as Australia waits for American nuclear submarines to be handed over in the early 2030s before the new AUKUS class nuclear subs start joining the fleet in the 2040s.
But there could be "a degree of overrun" during the works on the Collins class, defence officials admitted.
The works were some of the most complex conducted on the Collins class submarine since the build, the vice admiral said.
The fact the works were being done on subs close to the end of their life span meant a full schedule wouldn't be able to be mapped and the associated risks outlined "until we lock the actual scope down".
But there were contingencies in place for each extension and the availability of the Collins fleet hadn't fallen below 90 per cent.
"Notwithstanding the risks involved, I am very comfortable about where we stand at the moment," Vice Admiral Hammond said.
