Trailer Over Width

Get up to speed with the latest towing requirements for trailerboats

You’ve bought the boat, got the licence to drive it, and know all the rules about how to move it around safely on the water. Or so you thought. Pinged! In some areas, authorities are cracking down on oversized trailerboat rigs, while in others departments like insurance you may pay an even bigger price for towing illegally.

Getting the boat to and from the ramp can be a whole new world of regulation, and unfortunately, you’ll have to get your head around the rules to stay legal in all State.

Sorting out the towing rules and regs isn’t child’s play. We’re still waiting for responses from the ACT and Tasmania, while the NT’s efforts were lame.

However, many of the towing mistakes made by trailerboaters make are elementary and stem from not knowing the terminology.


First, you’ll need to learn weight definitions that are crucial for towing legally. So let’s start by getting the terminology right so we’re all on the same and legal page.

Tare mass (TM) is the mass of the trailer unladen, as it leaves the factory, ready to be registered and used. TM does not include the boat or options fitted (except factory-fitted options).

Gross Trailer Mass (GTM) is the mass that can be carried by the trailer’s wheels, with everything on the trailer (the boat, outboards, fuel, your fishing gear, the works).

Tow Ball Mass (TBM) is the amount of the trailer’s mass imposed on the towball of your tow vehicle.

Aggregate Trailer Mass (ATM) is the combination of GTM and TBM. Imagine your trailerboat, all loaded up but sitting on its jockey wheel — that mass sitting on the road is ATM.


Say your trailer has a 700kg Tare or TM, 3250kg GTM and 3500kg ATM. Subtract Tare from ATM, and you’ve got your maximum payload figure — 2800kg in this case.

If your trailer, your boat and everything in it weighed exactly 3500kg, you would have to have at least 250kg on the towball, because you can’t exceed the GTM figure in this hypothetic scenario of 3250kg.

Be aware that if you add extras to your trailer (such as an electric winch) that weight will become part of your payload.

The trailer compliance plate (for all trailers built since August 1989) will give rated maximums for both GTM and ATM, and in some cases TBM.

The only way to really know what your trailerboat weighs is to load it up as you normally would and put it over a public weighbridge. It might be the best $30 you spend.


You need the right tow vehicle and towbar. Both the vehicle and the towbar should have a plate or sticker noting maximum permitted towing mass figures, both for total towing mass and TBM (the vehicle towing capacities are usually on a sticker fitted to a door jamb. If not, these specifications will be in the owner’s manual).

If you bought a used vehicle with a towbar already on it, check that towbar capacity matches the vehicle’s towing capacity. Some don’t: for example the VFII Commodore can tow 2100kg but Holden offers a choice of 1200kg, 1600kg or 2100kg towbars for the VFII. Check the towball too: if you’re towing 3500kg then a 2000kg-rated towball is no use to you.

A vehicle’s legal TBM maximum is usually 10 per cent of its towing capacity: for example, with a 3500kg towing capacity, you’d be able to plonk up to 350kg of that on the towball. It’s not always as much as 10 per cent though, particularly with some Korean and European vehicles. You might find such a vehicle has 2500kg towing capacity but just 100kg maximum TBM.


Vehicle weight maximums include kerb mass, axle mass, Gross Vehicle Mass and Gross Combined Mass.

Subtract kerb mass from GVM and you have your payload — that’s the weight of everything in and on your vehicle (kerb mass always includes the vehicle’s fluids and depending on the manufacturer, a full tank of fuel and weight of a driver, which the manufacturer will stipulate in the owner’s manual).


Between 750kg-2000kg GTM, trailer brakes are necessary, but they can be override brakes and only need to activate on one axle.

In Queensland, NSW, Victoria, NT, SA and the ACT you can drive at up to the prevailing speed limit when towing (up to 110km/h).

In WA and Tasmania, the maximum legal towing speed is 100km/h, where permitted, while in NSW you’re restricted to a maximum of 100km/h if your combination’s GCM is more than 4500kg.


Before you even get to the state-by-state rules on towing speed, there are some vehicle manufacturers that cap maximum speed when towing. For Subaru, for example, it’s 80km/h.

With a GTM of between 2000kg-4500kg, the trailer must have brakes on all axles that can be controlled by the driver in the cabin. That means electric/hydraulic brakes for most boat trailers operating in this weight range.

The trailer also must have a break-away braking system, which automatically activates the trailer brakes if the trailer becomes decoupled from the vehicle. In NSW, it’s also a legal requirement to have a break-away battery monitor placed in view of the driver.

Once you reach 4500kg GTM, you’re into heavy vehicle class and will need a more sophisticated braking system such as air brakes.


Under VSB1 (the national standard applying to trailers), a trailer (including in this case, the boat) has to measure no more than 4.3m high and its rear over hang must not be more than 60 per cent of the wheelbase measurement (coupling to wheelset) or 3.7 metres, whichever is the lesser.

Maximum trailer width is 2.50m nationwide, which is fine for Aussie boats but can put owners of some US boats over the limit. Many larger US boats have a 2.59m beam width, pushing them into the oversize category. To tow such a boat, the rules are slightly different in each state.


In all jurisdictions, for boats no more than 2.90m wide, you’ll need ‘Oversize’ signs on the front of the tow vehicle and the rear of the trailer, and four flags on the trailer, all of which need to be to Australian Standards specifications.

NSW, Queensland and Victoria share a few rules relating to oversize trailerboats. The tow vehicle’s low-beam headlights must be on during the day (except NSW, where Daytime Running Lights – where fitted – are acceptable).

You can’t travel in poor-visibility conditions (fog, heavy rain, smoke) or other conditions where visibility is less than 250m. You can’t tow closer than 200m behind another oversize vehicle except when overtaking or in an urban area.

Tasmania shares all the rules above except for the 200m distance behind another oversize vehicle rule. There are time and place restrictions for oversize towing, which you can see in this document here.

In South Australia, you can tow up to 3.50m width under the ‘code of practice for the transport of indivisible items’ but not at night and no travel between 7-9am and 4-6pm Monday to Friday in the metropolitan area. Or you can get a $78 annual permit (plus a one-off $51 for the vehicle listing) to have no travel time restrictions and with the permit (for light vehicle, that is, less than 4.5 tonnes) you can travel on all SA roads.

For Western Australia, you’ll need a Class 1 Restricted Access Vehicle (RAV) Oversize Boat Transport Period Permit. Permit cost is $50 for 3 years. Oversize trailerboats up to 2.70m wide can’t be towed on freeways; wider than that and you have further restrictions. The boat trailer must have side marker lights no more than 2m apart.

As for NSW, oversize trailerboats up to 2.70m width cannot travel between sunset and midnight and can only travel between midnight and sunrise in certain areas.

There are numerous times and places when you cannot tow an oversize trailerboat in NSW. Go to this towing road access document for NSW for the full run down. If you started your trip before dark, you can drive up to 10km after dark before having to pull up.

Finally, a copy of the NSW Light Vehicle Agricultural and Load Exemption Ministerial Order 2015 (printed or electronic, so long as it’s legible) must be carried with you to show to the police if you’re pulled up.

In Queensland, at night, a yellow rotating warning light must be displayed. The maximum speed limit when towing an overwidth boat is 90km/h outside built up areas.

Oversize trailerboats can’t be towed at particular times or places in Queensland, and you can find the complete list here.

For Victoria, overwidth boats are classed as Class O vehicles under the Road Safety (Vehicles) Regulations 2009 and can be up to 3.50m wide. Oversize boats must have a yellow flashing safety beacon when travelling at night.

There are time/place exclusions for Class O vehicles, the detail of which you can see here.

Authorities in the ACT and NT had not responded with the requested oversize information by the time of writing. We’ll update with oversize requirements in those jurisdictions when the information arrives.

But towing a big or oversized boat across borders and interstate does require a thorough understanding of the rules and regulations. With more and more trailerboats being imported these days, it’s imperative we’re all up to speed.

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