And if it's not registered, that also means it's uninsured. It's a disaster waiting to happen - especially if you happen to be on the receiving end.
If somebody runs into you in an unregistered car, they aren't covered with compulsory third-party injury.
So if you're hurt, and they're broke, you're pretty much out of luck.
The cops use clever computerised cameras to nail some of them but many punters persist in taking the risk.
In California, they have solved the problem simply by making car owners attach their rego tag to the rear numberplate which has the rego month imprinted in the top left hand corner of the plate.
The `tax' tag goes on the right and is colour co-ordinated. If they don't match or the tag is missing, you are in trouble. So simple it would work in Oz.