Regarding some form of AEW for RAN assets, they would definately have their uses, even without there being a Fleet Air Arm attached. Please, lets not start that whole discussion again, even in jest, its just too soon.
Granted, the current plan is for Oz-based assets like JORN, Wedgetail, and likely the Poseidon, to provide surveillance support to any nearby RAN assets or task forces. Having said that, such assets might not be within range for proper support by external assets. This is where having some for of AEW would really come into its own. Also, keep in mind what AEW is, Airborne Early Warning... By having some form of sensor system at high altitude, the visual and radar horizons are extended beyond what conventional ship-mounted radars can 'see'. Whether the early warning delivered is of inbound flights of aircraft or missiles, or a possible hostile task force, just having such a piece of kit can reduce the 'fog of war' and potentially enabling unit/force commanders to make better, and certainly better informed, decisions.
Now, while a dismounted AEW system like something mounted on a Fire Scout UAV is interested and potentially something to pursue in the future, everyone seems to be overlooking a piece of kit that the RAN is already having enter service in ~2015-2016. Namely the MH-60R 'Romeo'. Flight ops and testing the Gulf of Mexico already determined that the Telephonics APS-147 search radar is able to monitor and track surface and airborne contacts. Granted, the radar is not optimized for the AEW role, and the naval helicopter will also have other duties, it is something which the RAN will have in service and could put to such use if/when needed.
What could be interesting is whether the RAN finds such a feature useful or important enough to have dedicated AEW aboard ship, and what modifications the RAN would like to such systems to enable them to be 'better' AEW platforms
-Cheers