I am with you Fathom. While well intended I read all the advice with a level of amusement. I used to fill the car up with petrol, buy a bottle of water and hit the road.
The shortest and fastest way from Sydney to Kununurra is via the main highways. 3 days will do you nicely. Its no big deal, petrol stations are nicely spaced. I would guess no further than 350 kms without checking. Having said that throw a jerry can on the roof racks just in case you miss judge a stop. I did that on my last trip and had the low fuel light on 110kms west of Isa, I didn't think I would make it. While there will be no traffic jams there are plenty of other people on the road so I wouldn't be wasting money on sat phones, UHF radios EPIRBS or the like. If you have mechanical issues pull over, put your bonnet up and wave down a passing motorist. Its pretty simple really.
Join NRMA/RACQ or the WA equivalent road side assistance just in case, that may save you a few bucks.