As I understand it, the Australian Navy patrols a relatively small portion of the ocean near their home continent. Do they have any reason at all to need three-month patrol capability?
Diesel-electrics submerged are quieter for a short time at any significant speed, or a couple days at very low speed, while nuclear subs are nearly as quiet, at speed, for much longer, without a tactical time/speed tradeoff. This is why pretty much everyone who can afford them uses nuclear attack subs in preference to diesel-electrics.
A I understand it, one important consideration is, if you might want to shadow targets during a crisis because orders might come in to take them out, you need to be able to do extended quiet operations without compromising speed to make that work.