IMO we have a window of opportunity with CEAFAR to increase the size and capability of the fleet over an extended period if we are smart about our procurement now.
Basically from my understanding Melbourne and Newcastle are much newer and in better condition than Sydney and Darwin so could be life extended more effectively than the older pair. The ANZACs are currently being upgraded to the state of the art with ASMD and other improvements,with the AWDs coming online over the remainder of the decade the RAN will have a highly capable fleet of between 11 and 13 surface combatants until the mid 2020s.
The key issue is the fact that the ACPBs are not combatants, are not blue water ( or possibly even green) water capable and are shagged. What they are replaced with will shape the capability of the fleet for decades to come.
If the ACPBs are replaced by new improved in shore PBs then the RAN will need to continue to deploy primary surface combatants in border protection roles. If the PB replacement includes several OPVs in addition to inshore PBs the RAN will no longer have frigates deployed as oversized OPVs. Should the ACPBs be replaced in their entirety by OPVs then the RAN would be able to deploy them further afield supporting regional missions and initiatives with our allies and neighbors.
The next step up (and least likely) is a mix of OPVs and light frigates or corvettes. Initially several OPVs would be built to complement the ACPBs allowing them to be withdrawn into the coastal or inshore patrol mission they are better suited to. Then the OPVs will be followed by a class of capable surface combatants that would literally be equivalent to an ASMD upgraded ANZAC but more modern, stealthy and cheaper to own and operate due to a smaller crew and more modern systems. These ships would be suitable for deployment to support the OPVs in border protection, work with friends and allies within the region and also undertake international developments to the Persian Gulf, counter piracy patrols off Africa etc.
This last,frigate /corvette option would permit the RAN to replace their remaining FFGs and the ANZACs with high end GP surface combatants from the mid 2020s without having to worry about them filling any of the current BP or patrol missions that take up so much of the current fleets resources. They could all, assuming money and technology was available be air warfare capable, ABM and cruise missile armed Type 26 or FREMM derivatives. They could be an indigenous design based on the light frigate experience, they could be a mix of DDG1000 and Hyugas.
Basically I am suggesting build OPVs and light frigates now to free up our options to replace the current frigate force in the future.