Whats wrong with using the current collins class hull mated to whatever new propulsion/electronics so as to bypass a new unique to australia or off the shelf european design for australias future subs?
There is no guarantee that the size, shape or composition of the current Collins-class SSG hull will be applicable or appropriate for the follow-on class.
One potential issue which comes immediately to mind if the
Collins-class hull were to be used, is where would the control systems for the TLAM go? The current combat data system used aboard the
Collins-class SSG is the same as used aboard USN
Virginia-class SSN's, except that the SSG does not have the stations to fire TacToms. IIRC part of the reason why the stations were 'left out' was that there was insufficient room within the
Collins-class for the stations to fit. In a related vein, since the future SSG is expected to have long-ranged land attack as one of its mission roles, it might be nice for the future sub to have some VLS capable of launching TacToms and/or Harpoon.
That way, valuable internal space for torpedoes, additional missiles or mines, etc is available while still allowing the sub to perform the mission.
-Cheers