Registration charges for vehicles take account of weight factors for vehicles because of road wear costs associated with heavier vehicles. Registration charges for vehicles under 4.5 tonnes Gross Vehicle Mass (GVM) are referred to as “light vehicles” and pay in accordance with RTA charges. Vehicles over 4.5 tonnes GVM are referred to as “heavy vehicles” and dealt with under National registration charges.

See the RTA website for details of registration charges

Inspecting these figures, we note that the “base charge” for lightweight cars under 975kg is currently $187 and a motorcycle is charged $87. This would indicate that the maximum envisaged weight for a motorcycle is 453kg.
This would be a very heavy motorcycle.
Most are under half that weight, although many are about three quarters of that weight when fully laden with pillion and holiday baggage. Few indeed would ever achieve this figure.

Motorcycles are “single track” vehicles and are not confined to utilize only the left and right hand wheel tracks in a narrow lane, so they distribute wear fairly evenly across the road surface. Road wear calculations for multi axle vehicles includes “tyre slip” induced pavement wear associated with turning, as well as gross weight. See the RTA Road Design Guide.

National heavy vehicle charges reflect the effort to recover the road expenditure associated with heavy vehicles.
Information Bulletin April 1998, National Heavy Vehicle Registration Charges:-
“This expenditure is recovered on the basis of the road wear costs caused by different classes of heavy vehicles, using average distance and mass for each class.”