Australia's fleet of trouble-plagued Collins Class submarine is available for operations for barely half the time of comparable countries' navies, a long-awaited independent report has found.

Part two of the Coles report, released this morning, concluded that ''the availability performance of the Collins Class has been slightly over half that achieved by the comparable international programs''.

Collins Class Submarine. Credit:Petty Officer Damian Pawlenko

Led by British expert John Coles, the report stated that this was ''significantly below the RAN's requirement'' for 2011-12.

The six Collins class submarines in Australia's fleet have been plagued by maintenance problems since the first one was launched in 1996. The government is considering various options to replace the fleet, including off-the-shelf, foreign-made subs, custom-made foreign subs or a fleet designed and built here in Australia.