Braking Systems & VSB1 Compliance
Braking requirements for light trailers in Australia are set by weight threshold, not trailer type. The rules below apply to box trailers exactly as they apply to any other light trailer under 4.5 tonnes ATM.
The Governing Standard
Trailers with an Aggregate Trailer Mass (ATM) of 4.5 tonnes or less are covered by Vehicle Standards Bulletin 1 (VSB1), the National Code of Practice for light trailer construction — currently Revision 6. Note the key distinction VSB1 uses for braking: requirements are set by GTM (Gross Trailer Mass) — the load actually sitting on the trailer's own wheels once coupled — not ATM, which includes the portion of weight transferred to the tow vehicle via the coupling.
Braking Requirements by GTM
| Loaded GTM | Requirement |
|---|---|
| 0 – 750kg | No brakes required |
| 751 – 2,000kg | Braking on both wheels of at least one axle |
| 2,001 – 4,500kg | Braking on all wheels, plus an automatic breakaway system |
The breakaway system requirement means that above 2,000kg GTM, the trailer must apply its own brakes automatically if it separates from the tow vehicle while in motion — normally via a breakaway cable connected to a charged actuator or battery-backed system.
Overrun (Mechanical) vs Electric Brakes
Mechanical override (overrun) braking — where the trailer's own momentum against the coupling activates the brakes under deceleration — is legal and fully compliant on Australian roads for trailers up to 3,500kg, with no electric brake controller required in the tow vehicle. Electric or electric-over-hydraulic brakes are an alternative above the 750kg threshold and are common on heavier or dual-purpose trailers, but are not mandatory purely because of GTM — overrun brakes remain compliant at any point within the 751–4,500kg range provided the breakaway requirement above 2,000kg is also met.
Questions & Answers
Does my box trailer need brakes?
Only if its loaded GTM exceeds 750kg. Under that, no brakes are required by law, though fitting them is never illegal or wrong — it's simply optional below the threshold.
What's the difference between GTM and ATM for braking purposes?
ATM (Aggregate Trailer Mass) is the total weight of the loaded trailer including what's transferred to the tow vehicle through the coupling. GTM (Gross Trailer Mass) is only the portion of that weight sitting on the trailer's own wheels. VSB1 sets braking requirements by GTM specifically, because that's the weight the trailer's brakes actually have to arrest.
Do I need a breakaway system?
Yes, once loaded GTM exceeds 2,000kg — an automatic breakaway system that applies the trailer brakes if it disconnects from the tow vehicle is a legal requirement above that threshold, in addition to braking on all wheels.
Can I legally use mechanical overrun brakes instead of electric brakes?
Yes — overrun (mechanical override) brakes are fully compliant on Australian roads for trailers up to 3,500kg and don't require an electric brake controller fitted to the tow vehicle.