This page describes the legal requirements in the Land Transport Rule: Vehicle Dimensions and Mass 2016 relating to the dimension and mass limits for heavy trailers (Classes TC (3.5 to 10.0 tonnes) and TD (over 10 tonnes), and heavy-vehicle combinations.

Also read Vehicle dimensions and mass for general dimension and mass limits and towing requirements.


The maximum allowable width (including the load) for full, semi, simple, pole, A- and B-train trailers is 2.55 metres (excluding side marker lights, direction indicators and the bulge towards the bottom of the tyre). The only extra width allowed is:

  • 240 millimetres each side for collapsible mirrors , and 1.49m when measured from the vehicle’s longitudinal centre line

  • the bulge towards the bottom of a tyre

  • central tyre inflation system hoses that extend not more than 75mm beyond the outside of the tyre on the drive axles

  • hubodometer assemblies that extend not more than 50mm provided it is fitted to a non-lifting, non-steering axle whose outer casings are of a light colour, provided the hubodometer is fitted on the axle that causes the least overwidth

  • cameras or close-proximity monitoring systems mounted on the side exterior of a vehicle that extends not more than 70mm from the side wall of the vehicle

  • devices for improving the aerodynamic performance of a vehicle that extend not more than 25mm from either side of a vehicle. 


The maximum allowable height (including the load) is 4.3 metres.

The height of the body or load of class TD trailers (those with a gross vehicle mass exceeding 10 tonnes) may be restricted by stability requirements – particularly the need for the vehicle to have a minimum static roll threshold (SRT). See Static roll thresholds for more.

All vehicles must be loaded in a safe manner, with a height appropriate for the type of load.

Ground clearance

Except when loading or unloading the vehicle, the minimum ground clearance for heavy trailers (ie those with a gross vehicle mass over 3.5 tonnes) is 100 millimetres. The ground clearance also has to be at least six percent of the distance from the nearest axle to the point where the ground clearance is measured.

Projecting loads

A load is allowed to project outside of the sides of a trailer, as long as the maximum allowable dimensions are not exceeded. See Vehicle dimensions and mass for information about the maximum projections allowed and the hazard warning devices you need to attach to projecting loads.

Load sharing

All axle sets in a heavy trailer must have a suspension system that provides effective damping and shares the load between the wheels of the set. No tyre should carry a mass more than 10 percent greater than the mass it would carry:

  • if all the tyres in the set were the same width and the load was shared equally between the tyres, or

  • if a tandem axle set contained a twin-tyred axle and a large single-tyred axle (‘super single’) and was built to divide the load between the tyres in the set in either the 60 percent twin/40 percent large single ratio or 55 percent twin/45 percent large single ratio. (A vehicle with these axle sets must have an indelible plate fixed to it by the manufacturer – for more information see the Rule.) 

Retractable axles – heavy trailers

Heavy trailers may have retractable axles in the drive axle or rear axle set if:

  • the retractable axle has an automated control to ensure that the axles that remain on the ground stay within the appropriate legal mass limits and manufacturer’s limits

  • forward distance and rear overhang limits are complied with both when the axle is on the ground and when it is retracted (Note that the position of the rear axis may change depending on whether the axle is retracted or in contact with the ground.)

  • such retractable axles are certified for compliance with the requirements in both the bullet points above

  • the vehicle must meet road user charge (RUC ) requirements. 

Rear steering axles – heavy trailers

Heavy full trailers, heavy simple trailers and heavy pole trailers must not have rear steering axles.

Heavy semi-trailers that aren’t part of an A-train or B-train may have a tandem axle set or tri-axle set with one steering axle.

A quad-axle set must have a rear steering axle that must be able to turn in both directions through an angle of at least 15 degrees.

A quad-axle set must be certified by a heavy vehicle certifying engineer or other vehicle inspector or inspecting organisation specifically approved by the NZ Transport Agency, to confirm that the steering axle meets the above requirement.

Stability of heavy trailers

Class TD trailers must have an SRT when laden of at least 0.35g (where ‘g’ is the acceleration due to gravity). SRT measures the stability of the vehicle. Class TD trailers with a body height or load height over 2.8 metres above ground must be certified that they have an SRT of at least 0.35g. (See Static roll thresholds).

More information

First, read:

Then read the relevant requirements from the list below: