The following description was taken from Overheight Vehicles – NSW Government:

Is your vehicle overheight?
A vehicle is overheight if, together with its load, it is more than 4.3 metres high. Special conditions apply to overheight vehicles in NSW. Know the height of your vehicle by itself as well as with its load before you leave the depot.

Why is there concern?
Vehicles travelling illegally overheight on NSW roads, bridges and tunnels pose a severe risk to motorist and heavy vehicle driver safety, in addition to threatening the protection of road assets.
Please ensure that you are on the right route and know the height of your vehicle – if in doubt, double-check.