RUC is not just for diesel vehicles either. If you happen to have a petrol powered vehicle over 3,500kg GVM then you have to buy RUC as well as petrol.
Large trailers also require a RUC.
Tractors and any diesel powered vehicle will have to pay RUC on the road, usually there is GPS monitored system to record the number of Kms a tractor does on the open road vs in the farm paddock.
With the changes coming in 2021 where RUC will be charged on electric vehicles, I believe that is the time to overhaul the system with a new weight band in for vehicles 0kg to 2000kg GVM then from 2001 to 3500kg GVM, to seperate the smaller vehicles from the large SUV/Utes. (this was on the drawing board back in 2005, but the Labour Government of the time elected not to enact it due to the loss of income). Also in 2021, taking all taxes off petrol and replacing with RUC will also capture hybrids who can buy petrol, but then run electrically, thereby become a cheaper way to run than pure electric with RUC.