Simple answer
Web Track use a feed from ATC secondary radar so any Mode C equipped aircarft show up. They have limited tag info. This is for noise monitoring stuff
The sites like Flight radar and Planfinder are all "plane spotter" type sites and they us private citizen ADSB / receivers who have a radio receiver and a data upload to the website of choice. You will see that each one has different coverage areas in Australia. In europe its almost 100%. Here not so much.
With the Flight radar you will mostly see RPT and RFDS movements, and the odd GA jet. There are about 50 GA aircraft in Aus now emitting ADSB. And if you knew where to look and what for......even a Retard Vehicle or two. One -10 in S.A. one in Qld
and a -7 out of YSBK.