it "depends"I'm unsure if this can be explained by those in the know in a public space, but
Are the power generation/drivetrain systems the main or majority area that need to be addressed if the Collins class boats are to go through for an additional complete refit? I vaguely remember hearing of a report that suggested that the boats could go through an additional refit cycle without structural risk, and that now that other systems are fairly well developed, a new drivetrain would complete a package that could go for another 10 years beyond current design life of type.
if Govt elected to go with a mid life upgrade/service life extension, then that would also imply that there was a preparedness to slip delivery of new replacement subs to the right
if we continue that assumption and that there is a willingness to extend Collins with such a mod, then that would come under the new timeframes for hulls out of the water (which almost reflect current nuke sub refurbs)
ie, subs under such a prog would be out of the water for (prob) 2-3 years.
major build progs to existings are (if new proposals get up) 3 year platform absences.....
at this point in time this is long distance and vague - so heavily wieghted on the "what options are there" magic fairy idea box.
there are 4 options, how they're firmed up is anyones guess