GBAS will always be necessary
GBAS (but not SBAS) will always be necessary, for sub-meter positioning, for some complicated technical reasons, to get the needed ANP, with integrity and continuity. And for some other technical reasons.
I'd suggest talking directly with Alex P or Pat R about any current issues being worked with particular equipment in Australia.
SBAS hasn't been needed since SA was set to "Off", and since we now have 30+ SVs active. The situation for not needing SBAS only gets better in the future, with yet more and better SVs on the way, with multi-constellation MMR or GPSSU ops on the way also, and with better tightly coupled/filtered inertial.