Current projects / Review of quad axle (policy work complete)

Investigating whether quad-axle group vehicles should be able to use higher mass limits without having to go through the performance based standards (PBS) application and approval process

Review of quad axle (policy work complete)

  • Scoping > 
  • Analysing issues > 
  • Analysing options > 
  • Implementing


This project is assessing whether it is possible to revise the mass limits for heavy vehicles fitted with quad-axle groups, without requiring the vehicles to have performance-based standards (PBS) scheme approval.

Specifically, we are assessing the potential to allow quad-axle groups in semitrailers and in the A trailers of B-doubles to operate at:

-        24 tonnes under general mass limits (GML)

-        27 tonnes under higher mass limits (HML) on suitable routes.

Next steps

The NTC recommended allowing quad-axle groups fitted to semitrailers and B-double combinations that comply with the axle spacing and road friendly suspension requirements to operate at up to 24 tonnes at GML and 27 tonnes at HML on available networks. Once implemented, this will make the gross combination mass of a semitrailer fitted with a quad-axle group 46.5 tonnes at GML and 50 tonnes at HML. Similarly, the gross combination mass of a B-double fitted with a quad-axle group will become 66.5 tonnes at GML and 72.5 tonnes at HML.

The increases to quad-axle group mass limits will be implemented by the NHVR, and each jurisdiction will identify networks suitable for these vehicle types.

The NHVR will consider during implementation what, if any, additional mass concession could be provided for heavy vehicles fitted with quad-axle groups that operate under Concessional Mass Limits (CML). The NHVR will then make a recommendation to ministers as to whether any further changes to accommodate vehicles operating under the CML scheme should be considered. 


This projects stems from a Transport and Infrastructure Council 2013 directive that asked the NTC to look into ways to encourage wider use of quad-axle groups.

A quad-axle group is an axle group in a heavy vehicle fitted with four axles. It is primarily used by road trains, B-double and semitrailer combinations.

Although these vehicles are designed to carry larger loads and do less damage to pavements than heavy vehicles fitted with tri-axles, they are currently only permitted to operate at the same mass limit (20 tonnes) under GML. In order to access the 27 tonne mass limit, the Heavy Vehicle National Law (HVNL) requires them to obtain approval under the PBS scheme.

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Project Manager Sri Kannan
Last Updated: 4/7/2017