The type of motorsport days most of us attend are multi-club sprints run by various CAMS-affiliated clubs (e.g. WRX Club, Alfa Romeo Owners Club, PIARC, etc), and whilst the events are CAMS governed, we don't use the CAMS racing categories (i.e. we don't have a class 'G'). SAU Vic club championship runs under it's own class system which you can read in the motorsport thread. As for other CAMS requirements, everything you need to know is in the motorsport championship thread.
http://www.sau.com.au/forums/topic/437292-2014-sau-vic-motorsport-championship/
If your intention is to enter actual CAMS organised series such as the Victorian Supersprint Championship or Improved Production, then CAMS categories will apply - you're best to ring them and ask your questions to get official answers rather than forums.