Honestly I'm surprised that Australia didn't go with nuclear submarines earlier. As an island nation, naval strenth needs to take top priority. It has an incredibly long coastline, much of which is sparsely populated, particularly in the north where any attack would be reasonably expected to come from, with few bases in the area to support fleet operations (to my knowledge there are no naval depots between Perth and Darwin with about 2500 miles of coastline in between). All potential adversaries are a long distance away and Australia has no overseas naval bases at which to refuel. Critically, all the major chokepoints that a potential adversary would need to sail through are very far from Australia, with it's territorial waters being extremely open. It's really a perfect use case for nuclear attack subs.