I would be terribly surprised if it was legal anywhere in Oz. They DO fine people in NSW for using a home-made plate the same as the car plate, on bike racks where the car plate is obscured. According to pushbike forums, the charge is "false" plates, 3 points and a whopper fine. They're supposed to either move the car plate to the bike rack, or buy from the gummint an extra (small) plate with the same number, to go on the rack.
And, of course, it has to be reflective, and lit at night.
Only reference on NSW RTA site seems to be a statement that vehicles must display "official" number plates. Guess that covers it??
You would think that unless something is specifically illegal, by regulation, then it is legal. Maybe the word "official" is the cruncher?
We must not interrupt the gummint's income stream..............