OTOH though, the RCN can field Sea Ceptor which is a smaller missile albeit with a comparable role as ESSM. IMO it would not make a whole lot to sense to fit SM-series missiles, ESSM/ESSM Block II, Sea Ceptor and a possible ER version which has similar range to ESSM/ESSM Block II.
Not sure ESSMII replaces SM.
However, even with ~50 km ranged air defence missiles, that is still only getting into a medium-ranged air defence missile, whilst MR and ER versions of the SM-2 can get out to between 70 km and 370 km, so SM-3 and ESSM/Sea Ceptor are really more complimentary capabilities.
Not sure SM-3 ESSM/Sea Ceptor is that complimentary. Pretty big hole between CAMM/ESSM and SM-3. Sm-3 is designed around exo-atmospheric threats.
However, to really make proper usage of such long-ranged area air defence missiles, some sort of AEW capability would likely be needed, though embarked naval helicopters with certain surveillance radars could provide some additional sensing.
Which is why having a sensor networks that are fully integrated with each other, space, OTHR, AEW, ship based sensors etc is so important. A lot of work has been put into making weapons able to find targets with minimal information. With OTHR you will know something is in the vicinity, or when something is being fired, but it won't provide accurate targeting for legacy munitions. But that is less important if your munitions are self guiding and fairly smart in capability. Some will be good enough to take out the launch platform (ship/plane) with minimal data, but intercepting the munition may be much harder as they are much smaller, stealthier, running more aggressive flight profiles .
Air platforms launching Yj-12 or the like would likely be a very challenging situation for a ship running a defence of just CAMM/ESSM. Sm-3 wouldn't be any help either. SM-3 doesn't even have an explosive warhead.
You also have issues of minimum engagement envelopes.
Australia is going to have issues deploying munitions like SM-3 and TLAM and perhaps advancements in SM-6.
Generally BMD requires multiple ships fusing data together with something like a AEW.
In the future it may no longer be possible to get enough of a radar horizon with just one ship. While missiles are getting longer ranged, they aren't really any faster.