Whichever class of ship/sub is best, i think the rudder has gone on this thread!
Australia is more than likely allowing itself, almost certainly through accident rather than design
, to become a nation that can quickly come to a nuclear position.
As has been pointed out, it has two strong nuclear allies (UK and US) and it might happen to be that Australia has no interest in nukes but that if a conflict looked like it needed some Nuclear weight on its side, it could have its own delivery mechanism for them. Even if it didn't have its own stockpile.
IMHO nuclear reactors for eneergy production are something all nations should retain an ability to build. I know there are weapons implications, but going forward there are also going to be power generation implications for most countries.