I agree that there are some quite combat capable corvettes, Germany's K130 readily comes to mind as an example.
However, the idea originally proposed was for a class of six corvettes in place of three
Hunter-class frigates, or a third of the planned frigates, or a quarter of the total number of major warships the RAN is planning on having.
Put another way, the proposal was to replace three blue water major warships, for a total tonnage of ~26,400 tonnes with six corvettes which are likely more suitable for littoral operations based off total size, displacement and armament, with a total tonnage of ~13,200 tonnes. Even if the corvettes were to deploy and operate in pairs, they would not realistically be able to provide the same level of capabilities that a single
Hunter-class frigate would be expected to. The one advantage that the corvettes might have, is simply being able to have a greater number of hulls in the water. Each of those hulls would be a quarter of the displacement of a
Hunter-class, which would dramatically impact what the corvette would be capable of doing.
There is a reason why the RAN and most other modern navies have demonstrated a trend towards replacing surface escorts with larger and more capable classes. The RAN is planning on operating a total of 12 major surface warships like it largely has for some time, but these will all be significantly larger and more capable than previous classes.
If the idea was instead to acquire some form of corvette either in addition to or perhaps in place of some of the
Arafura-class OPV's, that would be one thing.
In general terms, as one shrinks the size of a warship, the vessel tends to lose the properties which enable it to perform adequately in blue water operations. At the same time, the smaller sized vessels with lower displacement also have less "free" space and displacement available to use when fitting weapons, bunkerage, storage for victuals, etc. Again using the German
K-130/Braunschweig-class FSG as example, a range of 2,500 n miles @ 15kts is mentioned, but also an endurance of seven days without a tender. To provide some context, that would just about enable a K-130 corvette to transit (@15 kts) from sail from Sydney to Darwin without stopping en route to refuel and resupply, while staying within ~300 km of the eastern Australian coastline. Now for Germany, having such a short endurance would not be much of an issue, since the major nearby waters are the North Sea and Baltic Sea, where a corvette could easily return to a German port to refuel and resupply. A similar situation exists for the majority of other nations which operate advanced corvettes, the corvettes operate largely in littoral waters and could return to port comparatively easily. The situation for a navy like the RAN is quite different however, since port facilities around the Australian coastline can be few and far between. If RAN vessels needed to operate off the shores of some S. Pacific island nation, those already long distances could grow even longer, and the limited port facilities limited even further. If one then adds in RAN vessels needing a reserve of fuel and stores to permit the vessel to be on station for several days or longer...
A vessel like an
Arafura-class OPV could potentially manage this better despite being a bit smaller and with a lower displacement because the OPV would have so much less of that displacement taken up by weaponry and munitions, leaving more displacement available for fuel and stores.