Braking Systems & VSB1 Compliance
Braking requirements are set by weight, not trailer type — a loaded car trailer typically sits much heavier than a box trailer, which makes this the compliance question that actually matters most for car trailer buyers.
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. Braking requirements are set by GTM (Gross Trailer Mass) — the load sitting on the trailer's own wheels once coupled — not ATM. For a car trailer this distinction matters more than for lighter trailer types, since a loaded car trailer (trailer tare plus a full-weight vehicle) crosses these thresholds easily.
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 |
A typical passenger car weighs 1,200–1,800kg — meaning most loaded car trailers sit well into the 2,001–4,500kg bracket once trailer tare is added, requiring brakes on all wheels and a breakaway system as standard, not brakes on "at least one axle" only.
Overrun (Mechanical) vs Electric Brakes
Mechanical override (overrun) braking is legal and fully compliant on Australian roads for trailers up to 3,500kg, requiring no electric brake controller in the tow vehicle. Electric or electric-over-hydraulic brakes are common on car trailers at the heavier end of the range, where more precise, tow-vehicle-synchronised braking is preferred for towing a high-value load — but overrun brakes remain fully compliant at any point within the 751–4,500kg range provided the breakaway requirement above 2,000kg is also met.
Questions & Answers
Does my car trailer need brakes?
Almost certainly, in practice — most loaded car trailers (trailer tare plus a full-size vehicle) exceed 750kg GTM easily, making brakes a legal requirement, and frequently exceed 2,000kg GTM, requiring brakes on all wheels plus a breakaway system.
What's the difference between GTM and ATM for a car trailer?
GTM (Gross Trailer Mass) is the weight sitting on the trailer's own wheels once coupled — this is what VSB1 uses to set braking requirements. ATM (Aggregate Trailer Mass) is the total loaded weight including the portion transferred to the tow vehicle at the coupling.
Can a car trailer use mechanical overrun brakes instead of electric?
Yes, up to 3,500kg GTM — overrun brakes are fully compliant and need no electric brake controller. Many car trailer buyers choose electric or electric-over-hydraulic brakes anyway for more precise control towing a high-value load, but it isn't a legal requirement below that weight.