The following description was taken from NHVR: Vehicle Standards Guide 11 (VSG-11) :

The ability to readily see and identify a heavy vehicle, especially in low light and low visibility environments, has important road safety benefits. In the early 1990s, to improve heavy vehicle visibility, the Federal Office of Road Safety introduced a requirement for motor vehicles with a gross vehicle mass (GVM) over 12 tonnes and trailers with an gross trailer mass (GTM) over 10 tonnes to be fitted with rear marking plates.

Since this time, alternate approaches to improving the visibility of heavy vehicles have been developed such as the fitting of additional high grade retro-reflective material along the sides and rear of heavy vehicles (known as conspicuity markings). While not compulsory in Australia, voluntary fitting of additional reflectors or conspicuity markings to heavy vehicles is becoming more common and clearly shows the commitment of the industry to improving road safety.