From what I understand the ANZAC's weren't exactly originally intended to be the replacement for the DDG's and FFG's.
All 6 FFG's were also originally supposed to get FFGUP, which gave them 32 ESSM plus a combination of up to 40 missiles (mixed between Harpoon and SM2) from the Mk.13 launcher.
Originally there were only meant to be two FFGs upgraded with the first four replaced as part of a program to replace them and the Perth Class DDGs.
This was meant to follow on from the ANZAC program in Vic, and likely would have extended long enough to have eventually replaced the final, upgraded, pair as well.
We were that close to having a continuous build but in Victoria, not SA or WA. Ten ANZACs followed by eight or nine DDG and FFG replacements, followed in turn by ANZAC replacements.
Thus derailed when the government decided to insert the corvette program. Thus would have been for about a dozen ships with ANZAC like capabilities, but on a smaller hull.
To free schedule for this program the DDG replacement was deferred and the FDG upgrade was expanded to all six.
At this point I believe it was still intended to build nine ships to replace the DDGs and FFGs, there just would have been a period where there were fewer tier 1 ships while the tier 3 fleet was established.
The rest is history, corvettes were never ordered, Armidales were built, only four FFGs were upgraded and the ANZACs became the backbone of the fleet.
Only three Hobart's were built to replace the nine DDGs and FFGs, and the Hunters were ordered as replacements for the ANZACs.
It's only been since the Surface Fleet review that we are looking at increasing numbers again.