Australian light-trailer braking requirements are expressed using Gross Trailer Mass, not ATM. Under the current national VSB1 guidance, trailers at or below 750 kg GTM do not require brakes; above 750 kg GTM they require an efficient service braking system; and above 2,000 kg GTM they also require brakes on all wheels and an emergency breakaway system.
Those are thresholds, not a complete installation or maintenance specification. The applicable Australian Design Rules, state operating rules, approved components and manufacturer instructions still govern the actual trailer.
The threshold table
| Trailer GTM | VSB1 braking position |
|---|---|
| 750 kg or less | Brakes are not required; fitted brakes must still comply |
| Over 750 kg to 2,000 kg | Efficient service braking system required; over-run brakes may be used; brakes may operate on one axle |
| Over 2,000 kg to 4,500 kg ATM scope | Efficient service brakes on all wheels plus an efficient emergency breakaway system |
VSB1 Revision 6 covers trailers with ATM of 4.5 tonnes or less. It states that braking requirements are specified in GTM terms and that the underlying ADRs take precedence if the bulletin becomes inconsistent.
Why GTM, not ATM
GTM is the maximum mass transmitted to the ground through the trailer tyres when the trailer is coupled. ATM includes the load carried through the coupling when the trailer is uncoupled.
Because the brake system acts at the trailer wheels, the national requirements use GTM thresholds. Read the mass-definitions guide before applying a number from the plate.
Do not use an actual lightly loaded trip to argue that a trailer with a higher rated GTM belongs in a lower design category. The braking specification follows the applicable vehicle requirements and ratings.
At or below 750 kg GTM
VSB1 says brakes are not required on trailers that do not exceed 750 kg GTM. If brakes are fitted, they are not free from compliance or maintenance requirements.
The tow vehicle still needs to be rated for the trailer, the combination must be roadworthy, the load secured and every other limit observed. “No brakes required” does not mean stopping distance is unchanged or that any vehicle can safely tow the trailer.
Over 750 kg GTM
A trailer exceeding 750 kg GTM must have an efficient service braking system. VSB1 says that, except for over-run brakes, the system must be activated by a control signal generated by the tow vehicle.
Up to 2,000 kg GTM, over-run brakes may be used and brakes may be fitted to only one axle under the VSB1 guidance. The exact component, installation, adjustment and compatibility must satisfy the applicable rules and manufacturer instructions.
Drivers should test brake operation according to the controller and trailer manufacturers before travel. A connected plug or illuminated display does not by itself show that braking force reaches the required wheels correctly.
Over 2,000 kg GTM
Above 2,000 kg GTM, VSB1 requires brakes to operate on all wheels and an efficient emergency braking system. The breakaway system must automatically apply the trailer brakes if the trailer disconnects, and the brakes must remain applied for at least 15 minutes.
NSW Government towing guidance also states that a trailer over 2 tonnes GTM must have an automatic breakaway system, with brakes operable from the driver’s seating position.
The emergency function depends on components including its power source, switch, cable and brake hardware. Owners should follow the complete manufacturer test procedure and service schedule; pulling a cable casually can damage a switch or create an unsafe state.
Service brakes and breakaway brakes do different jobs
Service brakes respond during ordinary driving to slow the combination. A breakaway system is an emergency system intended to apply trailer brakes after separation.
A working service brake test does not prove that the breakaway battery can hold application for the required duration. A breakaway test does not prove proportional or correctly adjusted braking during normal use. Both require separate verification.
Safety chains are also separate. They do not replace brakes or the breakaway system, and the breakaway cable should be routed and attached exactly as specified so it can perform its own function.
Owner checks before towing
Use a system-specific checklist:
- Confirm GTM, ATM and VIN on the plate and RAV.
- Identify the installed brake type and approved components.
- Follow the brake-controller setup for the current load.
- Inspect plugs, wiring and visible brake hardware.
- Check any breakaway power-status indicator.
- Test service brake response at low speed in a safe location.
- Test breakaway operation only by the manufacturer’s procedure.
- Investigate warnings, weak response, pulling, heat or unusual noise.
- Keep service and battery records.
Do not crawl under an unsecured trailer or work on brakes without appropriate competence and support equipment.
Modification and replacement
Changing axles, ATM/GTM ratings, brake type, controller or breakaway components can affect compliance and certification. NSW modification guidance ties trailer braking systems to ADR 38 and may require certification when changes fall outside manufacturer instructions.
Use components with suitable approvals and ratings, installed as their manufacturer specifies. A larger-looking brake assembly or battery is not automatically a compliant upgrade.
Check the jurisdiction and current document
VSB1 is national manufacturing guidance under the Road Vehicle Standards framework. Registration, in-service rules and inspection processes are administered by states and territories. If travelling or registering elsewhere, verify the relevant road authority’s current requirements.
The reliable decision is not “my trailer is around two tonnes.” Read rated GTM from the plate and RAV, identify which side of each threshold it occupies, then verify that the complete system is compliant, maintained and working before departure.