Trailer Safety Chain: NZ Requirements, Sizes & Buying Guide

Hooking up the boat, caravan or work trailer feels routine—right up until the coupling pops off on the motorway. That’s why NZTA requires every light or heavy trailer to carry a safety chain (or certified cable) strong enough to hold double the trailer’s maximum towed mass. The chain’s link size, stamped rating and the way it’s attached must all prove, on inspection day and under real-world stress, that the trailer will stay behind the tow vehicle if the hitch fails.

This guide unpacks the current Waka Kotahi rules, shows you how to pick the right diameter and grade, walks through installation tips that inspectors like to see, and lists simple maintenance checks that prevent an unwelcome roadside drama. We finish with a buyer’s map of trusted New Zealand suppliers—so you can source a compliant, fairly-priced safety chain before your next trip.

Why Safety Chains Matter for New Zealand Trailers

A coupling failure is rare, but when it happens the results are ugly—skidding drawbars, smashed taillights and terrified motorists. A properly sized, correctly fitted trailer safety chain turns that potential chaos into an inconvenience by keeping the trailer tethered to the tow-vehicle long enough to stop safely. NZ Police log dozens of runaway-trailer call-outs each year; most involve either a missing chain or one that snapped. Imagine a boat trailer parting company halfway over the Auckland Harbour Bridge, or a laden farm trailer bouncing loose on a corrugated rural lane—both scenarios are avoidable for the price of a few metres of compliant chain.

Legal vs practical safety

Legally, your chain (or certified cable) must have a minimum breaking strength equal to two times the trailer’s maximum towed mass, and it must be attached with hardware of equal rating. Practically, seasoned towers add twin chains, cross them to form a cradle, and use high-grade shackles clearly stamped with their working load.

Consequences of non-compliance

Skip the rules and you risk an instant WoF or CoF failure, a roadside infringement fine, and insurers declining claims after an accident. In serious incidents, the Police can prosecute under the Land Transport Act, leaving you liable for damage, injuries, and hefty reparations.

NZ Safety Chain Regulations Explained (WoF & CoF)

Whether you roll into the testing station for a six-monthly Certificate of Fitness or an annual Warrant of Fitness, the inspector is reading from the same Waka Kotahi playbook—Vehicle Inspection Requirements Manual (VIRM) sections 8-1 and 8-2. In plain English the rules say every drawbar on the road must carry a secondary connection that can keep the trailer attached after a coupling failure and let the driver bring both vehicles to a controlled stop.

Light trailers (anything with a Gross Vehicle Mass up to 3,500 kg) must have at least one certified chain or cable whose minimum breaking strength equals twice the trailer’s Maximum Towed Mass (MTM). Heavy trailers over 3,500 kg follow the same 2 × MTM formula but usually rely on twin chains or a certified wire rope arrangement, and the attachment lugs themselves have to be proof-tested to the same figure. The regulations treat chain and attachment hardware as one system—if a bolt-on tab, weld or shackle is weaker than the chain, the whole setup fails inspection.

Waka Kotahi rule highlights

• Long-link chain or purpose-built wire rope only; no hardware-store ornamental chain.
• Each link must be manufactured, not welded or repaired after manufacture.
• Grade, diameter and manufacturer markings must be stamped or embossed and legible.
• Attachment points (weld-on lug or bolt-on plate) must meet AS/NZS 4177 and be located as close to the drawbar’s centreline as practicable.
• Re-welding or heat straightening damaged links is specifically prohibited.

Compliance testing and proof

Inspectors start with a visual once-over: is the chain present, free of corrosion, and long enough for full lock turns without dragging? Next they check for grade stamps, measure link diameter with callipers, and confirm the mounting tab or bolt pattern matches the chain rating. If documentation is available—mill certificate, supplier test sheet or a stamped working-load plate—it goes straight in the file and speeds the sign-off. Missing marks, unrated shackles or obvious pitting are red flags that can see your trailer safety chain fail the WoF/CoF on the spot.

Selecting the Right Safety Chain Size and Rating

One look at the rack of galvanised links in a hardware store makes it clear—“chain” is not a one-size-fits-all term. What matters is the stamped rating (minimum breaking load) and the link diameter, because NZTA inspectors will treat the lowest figure in your system as the true capacity. Remember the rule of thumb: your trailer safety chain must be able to hold at least twice the Maximum Towed Mass (MTM). Add a realistic safety margin for future gear you might throw on the deck, and you’ll only have to buy once.

Below you’ll find quick conversions and the little compatibility checks that stop headaches at WoF time.

Chain size–rating quick-reference table

*Assumes the legal 2 × MTM rule and rounded manufacturer figures—always verify the mill certificate.

Matching chain to tow coupling and shackle

A beefy chain is pointless if the shackle pin won’t pass through the link or, worse, is weaker than the chain. Check three things:

• Inside link height ≥ shackle pin diameter plus clearance.
• Shackle body and pin are stamped with a Working Load Limit (WLL) at least equal to the chain’s MBL.
• The tow-bar eye or lug is drilled or welded for the same pin size; avoid reducers or stacked washers.

High-tensile, gold-passivated shackles (Grade S or M) are the go-to for anything over 1,500 kg MTM.

Single vs dual safety chains

Legally, one correctly rated chain is enough on light trailers under 2 t if the drawbar has a certified chain bracket. In practice many owners fit two shorter chains crossed under the drawbar. The crossover forms a cradle that can catch the coupling and keep the drawbar from spearing into the asphalt, while the split load halves the stress on each chain and shackle. Heavier or long-drawbar rigs (e.g., tandem boat trailers) should default to twin chains for both compliance and peace of mind.

Materials, Grades and Link Types: What to Look For

Not all metal links are created equal. The chain you grab must shrug off salt-spray, bouncing corrugations and a sudden decoupling shock, so choosing the right material and link style matters as much as picking the correct diameter. Long-link hot-dip galvanised chain is the New Zealand default because the wider spacing lets a standard shackle pin pass through without grinding the galvanising off. For heavier rigs—big horse floats, tandem builders’ trailers—many owners jump to high-tensile Grade 70 or 80 chain: it carries more load for the same diameter and its gold or blue passivate coating resists rust almost as well as galvanising.

Stainless-steel chain tempts boaties, but 316 links can work-harden and lose strength if they thrash about on rough roads; they’re also pricey and may not carry the required grade stamp. Whichever material you pick, insist on paperwork and a visible mark that proves its Working Load Limit.

Understanding grades and markings

Grades describe the steel’s tensile strength—common stamps are L (Grade 43), 70 or 80. The number is usually raised on every 3–4 links; if the inspector can’t see it, you’ve effectively got an unrated chain. Match the grade to the shackle (S, M or marked WLL) and never paint over the stamps, as that obscures compliance.

When to choose cable over chain

Certified wire rope is legal but niche. Builders of large agricultural wagons or fifth-wheel caravans sometimes spec 12 mm galvanised cable with swagged thimbles because it coils neatly and weighs less than two 16 mm chains. The cable still needs a tag showing its breaking load and must attach via rated clevis fittings—not saddles or DIY crimps.

Correct Installation: Positioning, Attachments & Best Practice

Mounting a safety chain correctly is more than bolting on a few links and hoping for the best. The chain, attachment lug and shackle form a single fall-back system, so each part must be as strong as the others and positioned where it can actually work. Weld or bolt the chain to a certified tab right behind the coupling, keep the slack just long enough for a jack-knife turn, and make sure nothing drags. Anything less invites a WoF fail—or worse, a runaway trailer.

How to cross chains properly

With dual chains, swap sides before clipping on: left chain to right tow-bar hole and vice-versa. The resulting X hangs about 100 mm below the coupling, acting as a cradle if the hitch pops free. After fitting, do a full-lock test to confirm neither chain rubs the A-frame or roadway.

Choosing and securing shackles

Choose rated D or bow shackles clearly stamped with a WLL ≥ the chain’s MBL. Tighten finger-tight then nip an extra quarter-turn with a spanner; overtightening damages threads. Always secure the pin with a stainless R-clip or tie-wire so vibration can’t undo your only backup link.

DIY installation checklist

• Smooth-cut chain ends after trimming.
• Weld tabs: 6 mm fillet on 6 mm min plate, both sides.
• Bolting option: two M12 Grade 8.8 bolts torqued 85 Nm.
• Touch-up bare steel with cold-galv before hitting the road.

Inspection, Maintenance and Replacement Intervals

Give the chain a two-stage health check: a walk-around glance before every trip, and a deeper inspection every three months (or sooner if the trailer lives near salt water). Look for rusty pits, stretched or deformed links, missing galvanising, and attachment welds that have cracked. Use callipers to compare the worn link diameter with the stamped size—more than 10 % loss or a 5 % increase in inner length means the chain has stretched and must be replaced immediately.

Cleaning and corrosion prevention

After each salt-water launch hose the chain, shackles and tabs with fresh water, let them drain, then mist a light lanolin or CRC spray over the links—wipe off excess so it won’t drip on the driveway.

Record-keeping for commercial fleets

Fleets should log every inspection: date, odometer, inspector’s initials, findings and corrective action. A simple spreadsheet or notebook satisfies NZTA auditors and proves diligence if an accident investigation ever calls your maintenance routine into question.

Where to Buy Trailer Safety Chains in NZ (Retail & Online Guide)

You won’t struggle to source compliant chain—most marine chandlers, trailer-parts specialists and the big hardware chains stock pre-cut kits and sell long-link by the metre. Expect to pay roughly $8–$12 per metre for 6 mm, climbing to $25–$35 for 13 mm heavy-duty links. Regional hubs are well covered: Auckland’s industrial zones, Wellington’s marine precinct, Christchurch’s ag-supply stores, plus rural merchants who’ll courier nationwide.

What to look for when shopping

• Visible grade stamp and certificate
• Hot-dip or gold passivate finish ≥ 45 µm
• Rated shackles and matching mounting hardware included

Avoiding common purchasing mistakes

Skip ornamental chain, overseas grades with no test sheet, and hacksawing links—each move wipes out the rating and risks a WoF fail.

Quick Answers to Common Safety Chain Questions

• Is a safety chain compulsory in NZ? — Yes, on every trailer.
• Where do I hook it? — Onto the rated tabs on the drawbar, never the tow-ball tongue.
• How long? — Just enough for a 90° turn, not enough to drag.
• Padlock instead of a shackle? — Illegal; padlocks aren’t load-rated.
• Do break-away brakes remove the need? — No, the chain stays mandatory.

Wrapping It Up

A compliant trailer safety chain isn’t just a bureaucratic tick-box—it’s your last line of defence when the coupling decides to quit. Get the basics right: select a chain (or cable) with a stamped breaking strength of at least twice your Maximum Towed Mass, match it with rated shackles and lugs, cross twin chains where practical, and keep every link clean and rust-free. Add a 60-second pre-trip glance and a quarterly deep dive, and you’ll avoid WoF grief and, more importantly, a loose trailer careering into traffic.

Ready to upgrade or replace your chain? Drop into our Auckland store or browse the range of AS/NZS-compliant chains, shackles and mounting kits on the Action Outdoors homepage. Tow safe, tow smart, and enjoy the journey.