The point is, that if you have ADSB, you wont work with ADSB-2.
Australia ATC will have different ADSB systems, that is not really a problem. Cost is not an issue there. International aircraft will have ADSB-2.
ADSB aircraft will not see ADSB-2 aircraft and vice versa, that is the issue.
Many other features associated with ADSB, such as weather and trajectory are only available with ADSB-2.