Without drifting too far, JM. The signal is available. However, Australia would still have to provide a network of ref staions and a couple of uplinks to the satellite to transmit an augmented signal as a WAAS. The satellite is already visible to GPS receivers. There is already a ref station transmitting from CB as well as one from Hawaii. Nothing to do with state to state guarantees just the stations to guarantee signal quality. Got a link here somewhere.
As you can guess, this argument was done to death a while back. WAAS would be the single biggest improvment in aviation safety for IFR ops all round Australia. Every single aerodrome could have access to a CATI "ILS like" approach with glideslope information. This is what would have saved the likes of YHOT YLHR and YBLA more than anything else. A GLIDESLOPE!
There are a lot of pilots who would wish for this more than ADS-B!