The Wikipedia article on Australian licence plates highlights some special prefixes, but here’s a list I’ve tried to come up with that includes others they don’t show, from personal observations and gleaning information from the VicRoads web site.
AO (suffix) – accredited bus
CC – Consular Corps
E56 – trailers
HDV – horse-drawn vehicle
Mxx – state vehicles, such as (marked) police cars
M – metropolitan Melbourne taxi
U – Geelong, Ballarat or Bendigo urban taxi
PS (suffix) – peak period taxi
ST – substitute taxi (replacement for regular taxi which is out of service for repairs)
C – country/regional taxis
TOW (prefix ) / TT (suffix) – tow truck
VHA/VHB/VHC – hire cars (eg chauffeured)
S000 to S999 – hire motorcycles
I particularly like HDV.
Are there others?