Let’s face it: half the fun of owning a 4X4 is kitting it out with all the cool gear—bigger tyres, beefier suspension, a tough bull bar up front. You want to turn heads at the pub and tackle any track you come across. But there’s a sneaky little detail we often overlook: some modifications can make your beloved rig technically illegal. And if your 4X4 isn’t compliant, you could be staring down the barrel of hefty fines, defect notices, or even worse—having an insurance claim knocked back if something goes pear-shaped.
So, how do you know which mods cross the line? And what’s the deal with getting an engineer’s tick of approval? Buckle up. We’re about to dive into the must-know mods that usually need engineering to stay on the legal side of the tracks.
Why It Matters
The rules aren’t just there to make your life difficult, in theory at least. They’re about keeping you (and everyone else on the road) safe. While your flash new 4in lift may be full of quality gear that handles like a race car, flexes like Arni, and rides smooth and compliant over every undulation on and off the road, what about the P plater next to you with mismatched shocks that lift wheels, positive camber so it darts all over the road, and swaybars missing so he can barely keep it in a straight line. Unfortunately, the rules can’t easily tell the difference between the two, which is why engineering is the “figure it out” part of the equation.
1. Suspension Lifts Over 50mm
A suspension lift is one of the most common mods—who doesn’t want more ground clearance for rock-hopping and water crossings? But once you push that lift beyond 50mm, you’re probably in “engineer needed” territory. Why? Because raising your 4X4 changes its centre of gravity and how it handles corners, bumps, and off-camber sections.
Most states let you combine a modest tyre and suspension lift up to about 75mm without an engineer’s certificate (assuming you follow the exact guidelines). But if you’re chasing that sky-high 100mm setup, get ready to book an appointment with a certified engineer. They’ll look over your springs, shocks, control arms, and other bits to confirm that everything plays nicely together—and that you aren’t turning your rig into a wobbly monster on the road.
New South Wales (NSW)
- Maximum 75mm lift allowed without engineering
- Can be achieved through a combination of tyre diameter and suspension lift
- Example: 25mm suspension lift + 50mm lift from tyres is legal (but on the edge of legality)
- Anything above 75mm requires engineering approval
Victoria (VIC)
- Maximum 75mm lift allowed without engineering
- Must be a combination of:
- 50mm maximum suspension lift
- 25mm maximum tyre lift
Queensland (QLD)
- Same 75mm rule as Victoria
- 50mm limit for anything with Electronic Stability Control (ESC)
- Can increase lift up to 125mm with engineering approval
Northern Territory (NT)
- Allows up to 100mm lift over stock without engineering
South Australia (SA), Western Australia (WA), Australian Capital Territory (ACT), Tasmania (TAS)
- Capped at 50mm lift before requiring engineering approval
2. Bigger-Than-Factory Tyres
Slapping on oversized rubber is a rite of passage in the 4X4 world. Not only does it look tough, but it also helps with traction and approach angles off-road. However, tyres more than 50mm bigger than your original factory diameter often require an engineering sign-off.
Bigger tyres can mess with your speedo reading, increase your stopping distance, and alter your steering geometry—none of which is minor stuff. Engineers will check things like clearance (no rubbing on guards or suspension components) and whether your brakes and drivetrain can handle the extra grunt required to turn those chunky treads. It’s definitely worth the hassle, because the alternative is a potential canary sticker on your windscreen or a big question mark from your insurance company after a crash.
3. GVM (Gross Vehicle Mass) Upgrades
Keen to load up your 4X4 with a fully decked-out touring canopy, a rooftop tent, and enough gear to live off-grid for months? No worries—just be mindful of your legal load limit. A GVM upgrade bumps up the maximum weight your vehicle can carry. Since you’re altering critical components like suspension and possibly the chassis, a GVM upgrade always needs an engineer’s approval.
If you skip this step and get caught overweight, you could face fines and other nasty surprises. An engineer will confirm you’ve installed beefier springs and shocks and that your axles, brakes, and chassis can handle that extra tonne of gear. Think of it as the difference between a rig that’s merely tough-looking and one that’s actually up to the task of crossing the Simpson with a full load.
4. Seat Swaps & Mount
Upgrading seats in your 4X4 is a total game-changer for comfort—especially if you’re clocking up endless highway kays before hitting the dirt. But many folks forget that the seat mounts are just as important as the seat itself. If you’re swapping in aftermarket seats, or even second-hand seats from another model, an engineer needs to ensure the whole setup can handle the forces in a crash. A 100kg driver can exert near on 3T of force in a simple 60km/h crash
A dodgy mount could rip free in a serious bingle, sending you (and the seat) flying. And because seatbelt anchors often connect through or around the seat structure, the integrity of the seat and mounts is crucial. An engineering sign-off gives you peace of mind that your fancy new seats aren’t just comfy, but safe as houses.
5. Custom Bull Bars and Brush Bars
Bull bars are practically an Aussie icon, and they serve a genuine purpose: protecting your front end from ‘roos, wayward trees, and other off-road hazards. However, bull bars and brush bars also interact with your vehicle’s airbag systems and crumple zones. If the bar wasn’t designed to meet Australian Design Rules (ADRs), it could throw off how your car’s safety features perform in a crash.
Fitting a brand-name bar (like ARB) that’s designed specifically for your make and model often means it’s already ADR-compliant. But if you’re going custom or modifying an existing bar, get ready for an engineer’s review to make sure everything lines up correctly. That includes ensuring you’ve got proper forward visibility, no sharp edges, and correct airbag compatibility.
6. Chassis Mods
Extending the chassis for a longer tray? Shortening it for better departure angles? Adding extra cross-members to beef things up? Any major structural change to the chassis is going to require an engineer’s blessing. You’re not just messing with cosmetics here—this is the literal backbone of your 4X4.
Whether it’s for a bigger canopy, a camper conversion, or a wild custom project, a certified engineer will need to confirm the metalwork is up to spec, the welds are solid, and the modifications don’t compromise how your rig handles or distributes weight. Do it wrong, and you could face catastrophic failures off-road or on the highway.
7. Brake Upgrades
You can’t go up in vehicle weight, tyre size, or power without considering the extra strain on your stoppers. After all, what’s the point of a heavily modified 4X4 if you can’t confidently bring it to a halt? Swapping out brake calipers, rotors, or the entire braking system with non-factory parts often requires sign-off to prove the new components meet—or exceed—ADR standards.
An engineer will take your rig for a spin (and possibly do track testing) to confirm that your upgraded brakes perform properly under various loads. It’s not only about raw stopping power—balance and fade resistance under repeated heavy braking are key, especially if you’re towing or carrying weight on steep descents like down in the High Country.
The Bottom Line: Don’t Skimp on Engineering
Look, nobody wants to add extra steps or costs to their mod list. But getting an engineer’s approval is a small price to pay for peace of mind, legal compliance, and genuine safety. If you’re unsure whether a particular modification needs a certificate in your state, reach out to a professional engineer or check your local regs before the install. Trust us, it’s better to do it right the first time than to risk fines, defect notices, or insurance dramas down the track.
So go on—keep building that dream 4X4. Add the lift, the bigger rubber, the barwork, and the plush new seats. Just remember to keep it all above board. Because the only kind of trouble you want on your next trip is getting bogged in a mud pit, not fielding legal issues at the next RBT station.
2 comments
The one thing a lot of people forget is that any change to the suspension and wheels / tyres changes the angles and loads on your CV Joints substantially and can create major problems that provide you with no forward motion
Taking the third row seats out of your LandCruiser wagon ( to give more load space) can have problems with compliance for registration because it’s no longer a seven seat car as originally registered. Can have implications for insurance purposes.