Actually 9 hulls to 3.
These were originally meant to replace the 3 DDGs.
I think it's a bit of a stretch to say that it's actually 3 new hulls to replace 9.
You really need to go back a bit in history and see what came into service when and what replaced what at that particular time, and whilst some classes 'overlapped' not all hulls of those classes were in service during that 'overlap/transition' time too:
* 3 - Daring Class DD – 1957 to 1979 (Vampire continued till 86 in a training role).
* 6 - River Class (Type 12) DE – 1961 to (1985)-1998
* 3 - Perth Class DDG – 1965 to 2001
* 6 - Adelaide Class FFG – 1980 to – (2 decommissioned 2005 and 2008)
* 8 - Anzac Class FFH – 1996 to -
* 3 - Hobart Class AWD – expected from 2016 -
* 8 - Future Frigates FF(?) - expected from around 2027 (date may change with the new DWP and the Government looking to adapt the AWD hull for the frigate role and possibly start construction earlier, fill the 'valley of death' in shipbuilding?)
If you look at the period covering most of the 1970's, there was a destroyer/frigate fleet of 12, 3 x DDG, 3 x DD and 6 x DE.
The FFG's came into service, to the best of my knowledge, to replace the Daring Class (and cancelled DDL's), and as we know 2 FFG's were decommissioned and the remaining 4 were upgraded to take up some of the slack from the retirement of the 3 DDG's.
I suppose you can slice and dice it all different ways, and if you are looking for 'direct' comparisons as to what replaced what, I'd suggest that the 6 DE's were replaced by the 8 Anzac FFH's (an extra 2 hulls), the 3 DDG's are finally, many years late, to be replaced by the 3 AWD's (so that is one for one), so it's the 6 FFG's that are missing out on being replaced (in a way it's 4 hulls if you count the 2 'extra' Anzac's).
Still, by the end of this decade there will be 11 major fleet units, 3 x AWD and 8 x FFH, as opposed to 12 during most of the 1970's.
Will the last two Australian FFG's have their service life extended? Well I suppose that will be up to the Government of the day (and what is in the new 2015 DWP too) and what 'manpower' the Navy has to operate them and the material state of those two ships too, or if the Anzac replacement is brought forward, maybe there is more 'value' in retaining some of the 'younger' upgraded FFH than the last two FFG's??).
Maybe a more practical solution is to continue with the planned 11 major fleet units (3 AWD's, 8 Anzac/Future Frigates), and seriously look at a well armed and equipped fleet of OCV's to fill the gap between the top end and the PB's at the bottom end.
Would a fleet of OCV's be an appropriate replacement for the FFG's??
Cheers,