Here is a similar one VH-UUD doing circles on a training flight.
Not quite as prolific as your first picture but similar in nature
Operated by LEARN TO FLY AUSTRALIA PTY LTD
and in Flightradar24 for those who think it is better
We get the occasional plane with dummy credentials.
Registration is often TEST24 and ICAO number 123456 or 000000.
Often late at night there may be two or three but no location or speed and heading information and sometimes with location and apparently reasonable flight path.
Under normal conditions I can’t see planes on the ground at Melbourne, Essendon or Morrabbin airfields so I have no idea where they maybe coming from. Strange times for Mode-S only aircraft to be airborne and no sign of them on MLAT.