Ive never heard of a spudgun being registered. I beleive the have to be 'safe' in the eyes of the law and a plastic cannon would pass no such inspection [:0]. Plus, if its classed as a firearm its illeagal to build your own, if its viewed as a cannon and thought to be safe then you might have a chance. They do say a cannon has to be registered to be used but I beleive they mean black powder ones, spudguns may get through on a loophole though. Might be the difference when caught though of 'in possession of a banned firearm' to 'in possession of an unregistered firearm', thats if you have a gun licence.
Whatever anyone does though, dont quote me and if its to the cops dont mention me at all!;) Im only 70% sure anyway.
EDIT: One other thing. You may be able to register one here but you also have the licence catagory to own it. It may be registerable but not owned by you without getting a catagory 'E' licence.
All in all, its a joke (the 'safe' Australia gun laws).