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Extend a wheelbase, fit a crane, swap a suspension or re-rate a trailer, and you have modified a heavy vehicle. In Australia, most of that work has to be built to VSB6 and signed off by an accredited examiner before the vehicle goes back on the road. Skip that step and the unapproved modification tends to surface at the worst moment: a roadside inspection, a registration transfer or an insurance claim.
The tricky part is knowing where the line sits. A cracked crossmember welded back to factory specification is a repair. Moving that crossmember to suit a new body is not.
This guide maps the VSB6 modification codes to the fabrication and structural work we see most often in our workshops, and explains how certification works under the NHVR’s updated framework, which took effect when the amended Heavy Vehicle National Law commenced on 1 August 2026.
VSB6 is Vehicle Standards Bulletin 6: National Code of Practice for Heavy Vehicle Modifications. Published by the National Heavy Vehicle Regulator (NHVR), it sets the minimum design, construction, installation and performance requirements for modifying heavy vehicles. The current edition, Version 3.2, applies to vehicles modified on or after 1 July 2023.
It covers motor vehicles with a gross vehicle mass (GVM) above 4.5 tonnes and trailers with an aggregate trailer mass (ATM) above 4.5 tonnes. Its 15 sections run from Section A (Engines) to Section T (Tow Trucks), and each contains modification codes with a checklist the examiner works through at inspection.
Two rules from its introduction matter before any work starts:
Under the Heavy Vehicle National Law, a modification means adding or removing a component, or changing the vehicle from the manufacturer’s specifications. A repair that returns the vehicle to original specification using the manufacturer’s method is not a modification and needs no certification.
The grey zone is structural repair that departs from the manufacturer’s method, such as adding reinforcement plates or relocating crossmembers. Section H even lists chassis repairs within the H5 trailer chassis code, so raise any non-OEM repair with an examiner first. Our guide to trailer fabrication versus repair covers that decision for damaged trailers, and accident damage to a truck frame usually starts with chassis straightening and realignment back to OEM tolerance.
The NHVR Code of Practice for the Approval of Heavy Vehicle Modifications (Version 3.0, dated 1 August 2026) sorts every modification into one of three types, and the type decides who approves it.
No approval is required if the vehicle stays within the manufacturer’s specifications, still complies with the Heavy Vehicle (Vehicle Standards) National Regulation, and the work isn’t classed as common or complex. The NHVR’s heavy vehicle modifications guidance lists examples such as additional ADR-compliant lighting, aerials, air conditioning, alarm systems, and the manufacturer’s own wheel and tyre options where GVM and GCM are unaffected.
These are the routine jobs the bulletin covers. They must be built to VSB6 and approved by an Approved Vehicle Examiner (AVE), who issues a modification certificate to the registered operator and fixes a modification plate to the vehicle. No separate NHVR application is needed, and the approval is recognised in every participating jurisdiction.
Work that can’t be done to the bulletin, or that may take the vehicle outside a heavy vehicle standard, needs NHVR approval before it begins. The regulator may require an assessment by a registered professional engineer and, if satisfied, issues a Statement of Requirements. An AVE or state transport authority then inspects the finished work against it. Complex approvals carry an “H” prefix on the plate, so one involving a suspension substitution is listed as HF1.
The table maps the jobs operators ask us about most to the code an AVE will certify against. Separate design codes are shown in brackets.
Three details regularly catch operators out:
AVEs go by different names by state: Approved Persons in Queensland, VSCCS licensed certifiers in New South Wales, VASS signatories in Victoria, Recognised Engineering Signatories in South Australia and Approved Vehicle Certifiers in Tasmania. The NHVR’s Approved Vehicle Examiners page links to each scheme. Western Australia and the Northern Territory run their own schemes, and their approvals are recognised when the work follows VSB6 and carries a modification plate.
Our trailer fabrication work is planned around this process. Where approval is required, our team coordinates with qualified engineers and provides the measurements, drawings and test data needed for certification.
Often, yes. If VSB6 has a code for the modification, it must be approved even within OEM options. Minor modifications, such as optional accessories that keep the vehicle within specification, don’t need a plate.
Yes. Approvals from participating jurisdictions are recognised in all of them, so the vehicle doesn’t need re-certifying when it moves interstate. Western Australian and Northern Territory approvals are recognised if they meet VSB6 and carry a modification plate.
Only an accredited AVE approved for the relevant heavy vehicle codes can approve a modification. Some fabricators have AVEs on staff and others work with independent examiners. Ask who will certify the job, and for which codes, before work starts.
Treat it as unapproved until shown otherwise. An unplated modification can lead to a defect notice at roadside inspection and complicate registration and resale. Have an AVE inspect it: depending on what was done, it may be certifiable as is or need rework first.
VSB6 rewards operators who plan: decide whether it’s a repair or a modification, find the code, talk to an examiner, then build. Wales Heavy Vehicle Repairs handles fabrication, structural repair and accident repair for trucks, trailers and buses through a company-owned national network. If you’re planning a modification, or aren’t sure whether a repair has crossed the line, talk to our team before the steel gets cut.
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