Old Akro: WAAS is already here - but only for marine use! There are a number of land based, sea facing stations.
AMSA seems to be much better sorted than CASA.
There is presently no WAAS down this way. Probably differential GPS stations that you're talking about. Nothing to do with the relative abilities of CASA or AMSA. DGPS isn't all that practical for aviation due to the number of ground stations that would be needed.
The ground stations for WASS (SBAS) point at a geostationary satellite which rebroadcasts the correction signal to the GPS receivers.
SBAS when it comes to Australasia will be driven by government transport agencies like the Ministry of Transport here in NZ. It's reason for introduction will be multi disciplinary covering much more than just aviation. NZCAA, CASA, Airways, Air Services, will be small parts of a much bigger wheel.