Here is some infor that Austrlian car manufactures must follow to for ADR.
Had a look at the ADRs. ADR 61-02 says that for all vehicles other than L class (Passenger cars are MA class) the licence plate must be parallel to and forward of the front axel and no more than 1300mm from the ground. Subpart 2 of the ADRS (which contains all the definitions of terms) defines an axel as follows,
One or more shafts positioned in aline across a vehicle, on which one or more wheels intended to support the vehicle turn.
The above is the basis for all laws in each state.