Originally Posted by
Squawk7700
At what altitude does your ADSB kick in there with ATC?
ADS-B is a broadcast system that does not require interrogation from a SSR-interrogator, so that starts sending more or less immediately when you turn it on. (What you already know)
IF Airservices are using the tools available now (satellites), they could in practice see aircraft everywhere, as well as below line of sight coverage from the ground based surveillance stations. I'm not sure about the satellite coverage over Australia though, but it's being implemented in the North Atlantic as we speak.