My car runs on both fuels, Petrol and Lpg ,but gas being cheaper I will choose that option
The Eyre Highway across the Nullarbor Plain has the longest stretches between service stations (servo in Aussie slang). Between Norseman in Western Australia and ceduna in South Australia service stations are a minimum of 150kms apart and a maximum of 200kms. Fill up at each one.
Otherwise there are plenty of opportunities to get fuel.
https://www.worldnomads.com/travel-safety/oceania/australia/driving-across-australia almost 4 years ago