Well every dune is illegal but the risk of being caught and fined far out weighs the desire to just do it for the sake of it. I know of a few places where the authorities don't worry providing people don't treat the beach like 4wd'ers want to treat it but they are few and far between.
I can't remember the club down Portland way that is allowed to drive on the beach but if you're going to go that far you may as well just drive to SA and use their beaches.