I had a quick look on Vicroads site and Found:
"Where optional and non essential equipment is fitted such as air conditioning, rear window demisters, etc, it is not necessary for it to work providing it does not adversely affect the safety of the vehicle or its compliance with mandatory Standards for Registration."
No mention of needing a demister or heater of any kind.
You even need Demisters for CAMS
Looks like myself and my foreman are wrong . He has had us checking all cars rear demisters and A/C for RWC. I Can look through the RWC regulation books at work to see what they say as they are way more detailed then Vicroads.