Weigh-in-Motion · State Highway 1 · Bombay Hills
NZTA’s in-road Weigh-in-Motion scales now screen every heavy vehicle climbing SH1 at the Bombay Hills, and they read axle groups, not just the bottom line.
If you run freight on State Highway 1, the rules just got a lot harder to fudge. NZTA has installed Weigh-in-Motion (WIM) sensors on the approaches to the Bombay interchange, screening trucks in both the northbound and southbound lanes as they head toward the Commercial Vehicle Safety Centre.
Here’s the part that catches good operators out: these scales weigh you by axle group. You can be comfortably under your gross combination mass and still trip the system because one set, usually a drive tandem or a trailer tri-axle, is carrying more than its share. This guide breaks down the NZTA axle group weight limits that actually matter, explains how the Bombay Hills WIM enforcement really works, and shows how the Loadsense Titan Pro gives drivers the net and gross split on every group before they ever reach the weigh plates.
Part of NZTA’s national Commercial Vehicle Safety Programme (formerly Weigh-Right), the SH1 Bombay site pairs in-road weigh plates with number-plate recognition and electronic signage to pre-screen heavy vehicles at highway speed.
In-road sensors capture axle loads, axle spacings and gross mass as you pass, with no need to slow down or stop.
SCREENINGANPR ties the weights to your truck and trailer registrations and your vehicle configuration on record.
ANPR MATCHWithin limits, you carry on. Flagged as a potential overload, the VMS directs you into the safety centre.
CONTINUEA weighbridge reading confirms the offence and fines follow, and WIM data can drive RUC invoices 24/7.
FINE · RUCUnder the Land Transport Rule: Vehicle Dimensions and Mass 2016, three Class 1 limits apply at once: the individual axle limit, the axle set (group) limit, and the combined limit based on axle spacing. You have to satisfy all three. Loading every axle to its own maximum will usually overload the group.
Maximum mass on a single axle (or two axles less than 1 m apart), by tyre type.
| Axle / tyre type | Code | Class 1 limit |
|---|---|---|
| Single standard-tyred axle | S | 6,000 kg |
| Single large-tyred axle | SL | 7,200 kg |
| Twin-tyred axle | T | 7,600 kg |
| Single mega-tyred axle | SM | 8,200 kg (7,200 if steering) |
The group limit is often less than the sum of the individual axles, and it tightens as the axles sit closer together. These are the limits WIM checks most aggressively.
| Axle group | Typical config | Group limit |
|---|---|---|
| Twin-steer set | S + S (steer) | 11,000 kg |
| Tandem set, twin-tyred | T + T | 15,500 kg (≥1.8 m) |
| Tandem set, mega-tyred | SM + SM | 14,000 kg |
| Tri-axle set, twin-tyred | T + T + T | 18,000 kg (2.5–3.0 m) |
| Quad-axle set, twin-tyred | T×4 | 20,000 kg (3.75–4 m) |
Across the whole combination, the maximum mass rises with the distance from the first to the last axle, rising to the headline limits below.
| Distance, first axle to last axle | Max combined mass |
|---|---|
| 1.8 m to < 2.5 m | 15,500 kg |
| 3.6 m to < 4.0 m | 21,000 kg |
| 11.6 m to < 12.0 m | 36,000 kg |
| 16.0 m or more | 44,000 kg |
| 16.8 m or more · min 7 axles | 45,000 kg |
| 17.4 m or more · min 8 axles | 46,000 kg |
Picture a 44-tonne combination sitting bang on its gross limit. Shift the load 600 mm toward the rear and the gross hasn’t changed, but the trailer tri-axle is now 800 kg over. To the driver, nothing looks wrong. To the WIM scale, it’s a flag.
Dense product loaded a touch too far back overloads the rear group while the steer axle runs light, failing the 20% front-mass rule too.
As you unload across drops, the balance between groups shifts constantly. A combination that left the yard legal can go over mid-route.
Tankers and bulk tippers move their centre of mass with every hill and corner, leaving group weights you can’t eyeball from the cab.
Hook a different trailer and your spacings, and therefore your group limits, change. The number in your head no longer applies.
The Titan Pro is an onboard axle group weighing system built for New Zealand conditions. It gives the driver a live, per-group readout of net payload and gross weight for both the truck and the trailer, so they balance the load to the limit instead of guessing at it.
Pressure sensors tap the levelling valve on each air-suspended group; deflection sensors measure axle displacement on leaf-spring rigs, ideal for logging and construction work.
Steer, drive, and every trailer group displayed individually, with net and gross: the exact view WIM and the weighbridge see, before you reach them.
The central Integrator sends weights over RF wireless to the in-cab display and the mobile app, supporting up to two devices at once for driver and loader.
Loadsense streams every vehicle’s axle weights to a live cloud dashboard. Dispatchers and fleet managers see net and gross by axle group across the fleet in real time, spotting an out-of-limit group before it ever meets a weigh plate.
A red group on the dashboard means a quick call to the driver to rebalance, not a fine and an RUC invoice after the fact.
Confidence on every group means you load right up to the limit instead of leaving paid tonnes, and trips, on the table.
Time-stamped weight history backs your loading practice if a WIM-based RUC assessment is ever queried.
No. The WIM sensors screen vehicles in motion and flag potential overloads. A weight fine is issued from a static weighbridge reading once a flagged vehicle is directed into the Commercial Vehicle Safety Centre. Separately, NZTA uses WIM data to identify Road User Charges non-compliance around the clock and can issue RUC invoices from it, so a “clean pass-through” still matters.
A single twin-tyred axle is limited to 7,600 kg on Class 1 roads. A single standard-tyred axle is 6,000 kg, a large-tyred axle 7,200 kg, and a mega-tyred axle 8,200 kg (7,200 kg if it’s a steering axle).
A tandem set of two twin-tyred axles is limited to 15,500 kg when spaced 1.8 m or more apart, dropping to 15,000 kg (1.3–1.79 m) and 14,500 kg (under 1.3 m). A tri-axle set of twin-tyred axles tops out at 18,000 kg.
Yes, and it’s the most common reason compliant-looking trucks get flagged. Each axle, each group and the gross mass are capped independently, so an evenly-loaded gross figure can still hide an over-limit drive tandem or trailer tri-axle.
On air suspension it reads pressure at the levelling valve on each group; on mechanical suspension it measures axle deflection. The Integrator sends a live net and gross figure per group to the in-cab display and mobile app over RF wireless.
Get ready for SH1
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