A simple solution to the problem is to order and place a bicycle rack number plate down on the bumper, problem solved. This topic was done to death on more than one of the caravan/explore oz forums months ago.
If you are travelling through a State other than the one where your vehicle was registered the registration requirements of the home State apply.
Kedron made a kit up to lower the standard height of their number plates down to the number under the spares.
Cheers
Merv