You are correct, On Eyre, though all the farmers I know drive their diesel 4WDs both on and off road. How does one accurately keep track of the percentage on and off? Just a wild guess really.
You can also claim the rebate on petrol used in most of the same circumstances as diesel these days, too.
Only about 25% of the excise collected goes to the maintenance of roads. The rest goes into general Commonwealth revenue.
Marine transport also gets the excise fully rebated. Is there an excise on Avtur and Avgas? Damn right there is, but only for domestic aviation use. International flights are exempt. The Gillard government has legislated to just about triple the Avtur excise
from 3.6 cents in 2011 to 10.2 cents per litre in 2014-15. Somehow this is supposed to reduce carbon emissions.