To avoid excess wear on the road and to ensure that bridges and other road infrastructure can cope, there are limits to how much weight can be placed above different types of axle. The tyres on those axles transfer the weight through to the surface. The rules are anything but consistent.
 | Single axle, single tyres | Steer axle* Non-steer axle, tyres <375mm Non-steer axle, tyres 375-449mm Non-steer axle, tyres >450mm | 6000kg 6000kg 6700kg 7000kg |
| *Some exceptions exist in Queensland and for road trains | |||
 | Single axle, dual tyres | Pig trailer Any other vehicle A complying bus or a bus authorised to carry standing passengers Ultra-low-floor bus with no axle groups | 8500kg 9000kg 1000kg 11000kg |
 | Twin-steer axle group, single tyres | Non-load-sharing suspension system Load-sharing suspension system | 10000kg 11000kg |
| Tandem axle group, single tyres | <375mm 375-449mm >449mm | 11000kg 13300kg 14000kg |
 | Tandem axle group, dual + single tyres | Single on one axle, dual on the other A complying bus | 13000kg 14000kg |
 | Tandem axle group, dual tyres | Pig trailer Any other vehicle | 15000kg 16500kg |
 | Tri-axle group,single tyres | Single tyres <375mm, or a combination of singles and duals Pig trailer >374mm, duals on all axles or a combination | 15000kg 18000kg |
 | Tri-axle group, dual tyres | Vehicle other than a pig trailer, single tyres >374mm, or duals or a combination | 20000kg |
There are some associated rules:
Dog or pig trailers must not be heavier than the truck towing them.
The maximum GML (general mass limits) for a combination is 42500kg unless operating under a notice permit or specific scheme.
All heavy vehicle must be fitted with road-friendly suspension and accredited under the NHVAS (National Heavy Vehicle Accreditation Scheme).
Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists
