"Truck trailer" covers a lot of ground, from a single-axle tag trailer behind a rigid truck to a 26 metre B-double. If you're working out what to buy, or just trying to follow the conversation, the names can get confusing.
Almost every truck trailer on Australian roads can be sorted with two questions. How does it connect to the truck? And what shape is the deck? Answer those two and you know what the trailer is for.
This guide walks through both, then covers the common combinations, the mass limits that apply to each axle group, and how to narrow down the right trailer for your work. Each section links to our full guide on that trailer type if you want more detail.
Australian truck trailers fall into two groups by how they couple: semi trailers, which connect to a prime mover's fifth wheel by a kingpin, and drawbar trailers (dog, pig and tag trailers), which tow behind a rigid truck. Within each group, the deck type (flat top, drop deck, low loader or skel) decides what the trailer can carry.
How Truck Trailers Connect
Semi trailers
A semi trailer has no front axle. Its front end rests on the prime mover's fifth wheel through a kingpin, so the prime mover carries part of the trailer's load. Most of the deck types below, and every B-double, are semi trailers.
Dog trailers
A dog trailer carries its own weight on a front and rear axle group, and tows behind a rigid truck on a drawbar. The front axle group sits on a turntable, so it steers as the drawbar swings. A super dog uses a single front axle group with a Tandem rear group, which gives a tighter turning path than a standard dog. The two together are usually called a truck and dog.
Pig trailers
A pig trailer also tows on a drawbar behind a rigid truck, but its axles sit in one group under the middle of the deck rather than at each end. Like a dog trailer, it carries its own load. The NHVR caps both dog and pig trailers at a 1:1 tow mass ratio, so the trailer can't weigh more than the truck towing it.
Tag trailers
A tag trailer connects to the rear of a rigid truck through a fixed drawbar and hitch, with no turntable. Part of its load moves onto the truck's rear axle. Because of that, the NHVR allows a tag trailer a 1:1.3 tow mass ratio, provided the towing truck has at least 20% of the sum of the mass limits across its own axle groups and single axles.
Deck Types
Flat top trailers
A flat top has one flat deck for its full length, usually around 1050 to 1150mm off the ground. It's the general-purpose truck trailer: pallets, steel, building materials, machinery and anything else that fits under the 4.3m height limit with room to spare.
Drop deck trailers
A drop deck has a raised front section over the prime mover and a lower rear deck, typically around 850 to 900mm high. That lower deck lets you carry taller freight without going over 4.3m, which often means no oversize permit.
Low loaders
A low loader keeps one continuous deck as close to the ground as the coupling allows. It's built for heavy, tall plant like excavators, dozers and cranes, so low loaders are usually specced with tri or quad axle groups, and some combinations add a dolly to spread the mass further. For the biggest jobs, see our heavy haulage trailer guide.
Skel trailers
A skel trailer (short for skeletal) drops the deck altogether and keeps only the frame, with twistlocks to hold shipping containers. Less steel means less tare weight, so more of your legal mass goes to the container. Skels are a common sight between ports and depots.
Specialist trailers
Some jobs need a trailer built around one kind of load. Midland also builds tanker trailers, pole jinkers, hay spec combinations, beekeeper trailers and skip bin transfer trailers. Each one starts from the same question as any other trailer: what are you moving, and how do you load it?
Common Combinations
A B-double is a prime mover towing two semi trailers, linked by a fifth wheel on the back of the first (A) trailer. A full 26 metre B-double carries 62.5t under General Mass Limits, or 63.0t with a complying steer axle, according to the NHVR's 2026 Class 2 B-double operator's guide. Common B-double layouts run Tandem or tri axle groups on each trailer.
On the drawbar side, the truck and dog is the workhorse combination for bulk materials and civil work, and a rigid truck with a tag trailer is a common way to add capacity without stepping up to a prime mover.
Mass Limits by Axle Group
Whatever the trailer type, general access limits are 4.3m high and 2.5m wide. How much a trailer can carry comes down to its axle groups.
Single axle, dual tyres: 9.0 t
Tandem axle group: 17.0 t
Tri-axle group: 21.0 t
Quad-axle group (dual tyres, or single tyres 375mm or wider): 21.0 t
Source: NHVR, General Mass Limits (GML): Information sheet, published 11 August 2026. Higher Mass Limits and Performance Based Standards (PBS) approvals can lift these on approved routes. Our guide to PBS and the 2026 mass limit changes covers how.
Midland's Truck Trailers
Midland Pty Ltd manufactures open-top truck trailers at its factory in Kyneton VIC: flat tops, drop decks, low loaders, skels, tag, dog and pig trailers, B-doubles and the specialist builds above. Our Tow and Go trailers are a proven Midland design, finished in our factory and ready to work. Where a job needs something no standard trailer can do, we engineer the trailer from your brief.
How to Choose the Right Truck Trailer
What is your truck? A prime mover pulls semi trailers. A rigid truck pulls dog, pig or tag trailers. That one fact rules out half the options.
How tall is the load? If your freight sits well under 4.3m on a flat top, a flat top is the simplest choice. If it's close, look at a drop deck. If it's tall and heavy plant, look at a low loader.
How heavy is it? Work out the load mass, then match it to axle groups using the table above. Remember the trailer's own tare comes out of the same limit.
Where are you going? General access, Higher Mass Limits and PBS routes each change what you can legally carry, so plan the trailer around the routes you actually run.
Frequently Asked Questions
What are the main types of truck trailers in Australia?
The main types of truck trailers in Australia are semi trailers, which couple to a prime mover's fifth wheel, and drawbar trailers (dog, pig and tag trailers), which tow behind a rigid truck. By deck, the common types are flat tops, drop decks, low loaders and skel trailers. A B-double is a prime mover towing two semi trailers.
What is the difference between a semi trailer and a dog trailer?
A semi trailer has no front axle and rests on a prime mover's fifth wheel, so the prime mover carries part of its load. A dog trailer has its own front and rear axle groups, tows behind a rigid truck on a drawbar, and carries its own load. Under NHVR rules a dog trailer can't weigh more than the truck towing it.
What is the difference between a drop deck and a low loader?
A drop deck has a raised front section and a lower rear deck, typically around 850 to 900mm high, for taller freight under the 4.3m limit. A low loader keeps one continuous deck as low as the coupling allows, for heavy, tall plant such as excavators and dozers, and usually runs tri or quad axle groups.
What does Tandem mean on a truck trailer?
Tandem means an axle group of two axles. Under General Mass Limits in Australia, a Tandem axle group can carry 17.0 tonnes, compared with 9.0 tonnes for a single axle with dual tyres and 21.0 tonnes for a tri-axle group.
How long is a B-double in Australia?
A full B-double in Australia is up to 26 metres long and, under General Mass Limits, can run at 62.5 tonnes, or 63.0 tonnes with a complying steer axle. Shorter B-double combinations run at lower mass limits, according to the NHVR's 2026 Class 2 B-double operator's guide.
Talk to Midland
Midland Pty Ltd manufactures truck trailers at its factory in Kyneton VIC. Tell us what you're moving and how you load it, and we'll help you work out which trailer suits the job.
Related guides:
What is a Semi Trailer? Your Complete Australian Guide
What is a Drop Deck Trailer? Your Complete Australian Guide
What is a Low Loader Trailer? Your Complete Australian Guide
What is a B-Double? Configuration and Mass Limits