The legal requirements of gross trailer mass

If you’re towing a trailer and want to arrive safely with everything in good working order, it’s important not to overload the trailer. But how do you know when it’s over the limit? There are rule books available, but it’s not always clear what terms like ‘gross trailer mass’ or ‘maximum trailer mass’ actually mean.

Before we delve into maximums, let’s look at some of the jargon.

Any reference to trailer mass will inevitably include ‘GTM’, ‘ATM’ and ‘tare’, which are all ratings supplied by the manufacturer.

Aggregate trailer mass (ATM): Is the total mass rating of the trailer (unhitched) when it’s carrying the maximum load recommended by the manufacturer.

Gross trailer mass (GTM): Is the total mass rating that can be imposed on the trailer’s axle when the trailer is fully loaded and coupled to the towing vehicle.

Tare mass: Is the actual weight of the trailer. It will be stamped on the vehicle by the manufacturer.

Trailer ball load: Is the maximum allowable weight that a loaded trailer can impose on the tow bar of the towing vehicle.

For more detailed explanations, see our guide to towing capacity.

What is the maximum trailer mass?

Australia-wide, the allowable maximum mass for the trailer is whichever is least between:

1. The capacity of the tow vehicle’s towing attachment, or
2. The towing limit specified by the vehicle manufacturer for the tow vehicle.

In other words, the most you can tow is the amount specified by the vehicle manufacturer or the capacity of the tow bar – whichever is least.

If you want to know how much your vehicle can tow, check the owner’s manual.

What are the regulations?

A set of national towing regulations has been in place since 1998, governing towing in Australia for trailers up to 3,500kg aggregate of loaded trailer mass (ATM).

• All new trailers built since August 1989 are required to have a Vehicle Identification Plate (VIP) listing the trailer’s ATM, GTM and tare weight.
• If the trailer is not fitted with brakes, then the maximum mass must not exceed the unladen mass of the motor vehicle.
• All trailers with GTM exceeding 750kg must have brakes.

Owners of heavier vehicles such as four-wheel drives and light commercial vehicles should also take care not to exceed the gross combined mass (GCM) of the vehicle.

The GCM refers to the maximum that a vehicle plus its load, including trailer, is permitted to weigh. So if you have your teenage son and his mates on board, together with all their luggage and camping gear, the maximum allowable trailer mass may need to be reduced so that you don’t exceed the GCM.

Ultimately, it is the responsibility of the driver to ensure that they do not exceed any of the ratings, whether the trailer is partly or fully loaded. Apart from legal ramifications, a failure to comply could invalidate any warranty, nullify insurance and affect the vehicle’s safety and reliability.

Not sure how much weight you can carry in your trailer? Our experts have put together a guide that makes it all clear.