I live in Queensland, Australia, different state, but as far as I know the laws are the same Australia wide. I think the police officer was just trying to scare you, but who knows I could always be wrong. And as far as I know there are no (legal) ways to register these.
I ride my 66cc fairly reguarly and have had no attention from the police, but that doesn't mean my bikes legal. I just have to cross the wrong police officer on the wrong day and I could be on the receiving end.
I'm also just finished my 98cc Villiers build and have every intention of riding it, and whatever happens, happens. I just hope I get a little use before the boys in blue spoil my fun