Nuclear submarines will allow Australia to project maritime power far from our shores. This is critical for an island nation heavily dependent on trade by sea.
This is a counterpiece to Security without submarines: the military strategy Australia should pursue instead of AUKUS, which was published (and posted here) about a week ago.
I think the complexity of nuke subs is only justified if you also have nukes. They are ideal for a crew to go out with a complement of 16 nukes to loiter in the unknown depths for 6 months. They’re just out there as a deterrent to let your enemies know that Very Bad Things will suddenly happen to a few of their major cities if they want to try to lob a few nukes first.
Other than that particular purpose, they are quite a costly way to just go out and patrol your territory. That whole nuclear supply chain is ridiculously expensive if you don’t already have one set up for other nuke stuff. Drop that expense and you can get quite a lot more hardware for your money.