GB118,
I believe to convert a JAA CPL to an AUS CPL all that is required is for you to do a Flight Rules & Air Law exam, obtain a Class 1 medical (only available in Australia) and then a flight test with an Approved Testing Officer. There are plenty off ATO's around, and often large flying schools have them as CFIs or have a couple they use regularly for testing.
All of this is an awful lot cheaper and quicker than coverting an Aus CPL to a JAA CPL.