From our friends at the SAAA. They have a good plain english description of what is required:
GPS See AIP Gen section 1.5. GPS for IFR use must comply with TSO. A GPS compliant with TSO C129a2 can be used for enroute and terminal area navigation. C129a1 can conduct non-precision approaches. However to take advantage of lower weather minima a VOR or ADF need to be onboard and at the destination. See the AIP for details of operational and weather requirements for destinations and alternate airports with and without ground aids.
GPS compliant with the new TSO C146a are WAAS upgradeable and are expected to be approved for sole means of navigation for enroute navigation and also (in future) for precision approaches. CASA staff have confirmed that the US WAAS correction signal does cover Australia, and they will progressively introduce precision GPS approaches in the future.
Now just check which part applies to your equipment.....