I can rule out Astutes now, as the RAN is looking at something around Collins size and non-nuclear. Im happy to put doubt on 12 collins MKII, as it would need a bigger change in the fleet manning and use then what is currently underway.
Headquarters will always put more Anzacs on the surface then Subs in the water, the role and flexibility of a 'skimmer' is that much more different to a subs.
Navy has a role to defend australia, and protect the nations interest(its on every Navy car) The defence of the realm so to speak is provided by a mix of surface ships, subs and Air force assets. To then move away from Aus you require surface vessels. While keeping within the context of the topic, a carrier will need protection, this is provided by ships, subs can be utilised, but a ship is the number one defensive asset. You take that away so that you get 5 more aircraft on the carrier, then you lack proper coverage. Thats why when deploying a Aircraft Carrier they like to send them in a group, with a mix of units to allow coverage for the carrier to rely on its own task at hand, and the surface fleet to provide defence. What im getting at is why take away ship numbers in the name of a few aircraft This low cost approach does little for the ADF as a whole and in fact lowers its ability to respond to emergencys, all in the name of a few more pilots with wings.
There will always be a MWV fleet, and the better the numbers the better the coverage. They are cheap, somewhat reliable vessels that provide a strong "coast Guard" role in protect the northern area. By assigning a MFU you are wasting few, resources and sea time on an asset better utilised and trained up ready for deployments off the aus station. 10 F35Bs wont stop illegal fisherman, or save a yachtsman from sinking.