The maximum speed for a car towing a trailer is 90kph.
Speeds are restricted when towing a trailer for a number of reasons. Cars are not designed for towing. They have towing ability, but it is not their main function, therefore when they are towing a trailer the handling and braking performance is reduced.
The reduced speed limit takes into account that the extra weight of a trailer will increase the stopping distance (which is why you should leave a four-second gap to the vehicle in front when towing a trailer).
The trailer also exerts an extra sideways force on the rear of the car while cornering that can cause the car and trailer to jack-knife in extreme circumstances. There is more risk of this happening when the roads are slippery or if the vehicle is an unladen ute (i.e. the back end of the vehicle is light).
Cars and trailers represent a longer vehicle to pass for other road users. There are two opposing arguments here:
by restricting the speed you force other motorists to pass, but at least it's easier for them to pass
if we didn't restrict the speed then there wouldn't be the need to pass, but the risk of accidents would be higher.
An inadvertent benefit of this is that cars tend to be slightly more economical at 90kph than 100kph so you will enjoy fuel saving at the expense of time savings.
Read our complete guide to towing a trailer for more information.