VSB 6 Explained: Heavy Vehicle Modifications in Australia

Vehicle Standards Bulletin 6 (VSB 6) is the National Code of Practice for Heavy Vehicle Modifications. It sets minimum technical requirements for many common modifications to heavy motor vehicles and heavy trailers, but it does not replace the approval and registration process administered by the relevant regulator.

This guide explains what VSB 6 covers, when it applies and why a compliant design still needs the correct certification pathway.

Which vehicles does VSB 6 apply to?

For modification purposes, a motor vehicle is generally treated as heavy when its Gross Vehicle Mass (GVM) is greater than 4.5 tonnes. A motor vehicle does not become a heavy vehicle merely because its Gross Combination Mass (GCM) exceeds 4.5 tonnes.

Trailer classification is based on the applicable Aggregate Trailer Mass (ATM) threshold and regulatory scheme.

VSB 6 is relevant to in-service heavy-vehicle modifications. A new vehicle changed before first supply may instead require approval under the Commonwealth Road Vehicle Standards framework, including Second Stage of Manufacture where applicable.

What does VSB 6 cover?

VSB 6 Version 3.2 is organised into technical sections covering:

• engines and transmissions;

• tail shafts and rear axles;

• front axles, steering, wheels, rims and tyres;

• suspension and brakes;

• chassis and body mounting;

• cabins, seating and occupant protection;

• fuel systems;

• tow couplings;

• vehicle-mounted lifting systems;

• vehicle ratings, including GVM, GCM and ATM changes; and

• tow trucks.

Each section contains modification codes with a defined scope, technical requirements, evidence requirements and certification checklist. A project involving several systems may require several codes. Selecting one code does not remove the need to assess interactions with the rest of the vehicle.

Is VSB 6 itself a legal approval?

No. VSB 6 is a technical code of practice. It does not, by itself, approve a vehicle or update its registration record.

In Heavy Vehicle National Law jurisdictions, a common modification within an authorised code may be certified by an Approved Vehicle Examiner who is authorised for that code. A modification outside the code may require specific approval from the National Heavy Vehicle Regulator.

Western Australia and the Northern Territory administer heavy-vehicle modifications through their own systems. VSB 6 remains an important technical reference, but the certifier, documentation and administrative steps are determined locally.

Common versus specific modifications

A common modification is one that falls within an applicable VSB 6 code and can be assessed through the authorised examiner pathway.

A specific modification is outside the scope of the available codes or departs from their requirements. It generally requires an application supported by engineering evidence and approval from the relevant regulator before certification.

The classification should be confirmed before fabrication begins. Completing work first can leave the owner with components or geometry that cannot be approved through the intended pathway.

What evidence may be required?

The evidence depends on the modification. It may include:

• manufacturer specifications and body-builder information;

• component ratings and material certificates;

• drawings, calculations and load-path assessments;

• chassis stress and fatigue assessment;

• suspension, steering and axle-capacity evidence;

• brake calculations or physical testing;

• welding procedures and welder qualifications;

• installation photographs and staged inspections; and

• final mass, dimension and functional checks.

An engineering calculation is only one part of compliance. Installation quality, component traceability, testing and the vehicle's final configuration also matter.

Vehicle-rating changes under Section S

VSB 6 Section S addresses heavy-vehicle rating modifications, including certain changes to GVM, GCM and ATM. A higher rating cannot be justified solely by fitting heavier springs or tyres.

The assessment may need to consider chassis, axles, suspension, wheels, tyres, brakes, steering, powertrain, cooling, tow systems and stability. The proposed rating must also remain consistent with applicable legal mass and dimension limits; a technical rating does not automatically create road access at that mass.

Typical heavy-vehicle modification process

1. Define the vehicle, proposed modification and intended operating mass.

2. Confirm the registration jurisdiction and whether the proposal is common or specific.

3. Engage an appropriately authorised examiner or engineer before starting work.

4. Develop the design and evidence package.

5. Complete fabrication with required staged inspections and records.

6. Undertake the specified inspection and testing.

7. Obtain the certificate, plate or other approval evidence required by the jurisdiction.

8. Complete any registration or regulator notification steps.

The exact sequence varies with the vehicle, modification and jurisdiction.

VSB 6 and interstate operation

VSB 6 provides a common technical foundation, but certification authority and administrative recognition are not identical across Australia. If a vehicle will be registered in another state or territory, confirm the receiving authority's requirements before committing to the design or relying on an interstate approval.

Official references

• NHVR: Vehicle Standards Bulletin 6

• NHVR: Heavy vehicle modifications

• Commonwealth: Second Stage of Manufacture

Technical content reviewed 13 August 2026. Always confirm the current code version and jurisdictional process before modifying a vehicle.

KME Vehicle Engineering provides design assessment and certification support for heavy-vehicle modification projects. Learn about KME's heavy-vehicle engineering services.