As indicated above, will you be gtting a JAA PPL in Australia or a CASA PPL? And in which country are you thinking of doing your CPL and ground school?
I'll assume that you mean CASA PPL and want JAA CPL.
The only reason you may need to convert your PPL would be possibly for hour building in a JAA country with your CASA licence but that would depend on where, and from whom, you were hiring or using an aircraft.
Otherwise you only need an ICAO PPL to start ATPL ground studies.
Cheers
Whirls