Chris - Henry's right, Oz does have a tendency towards Bigness, and if he averages 3 days driving from Melbourne to Perth, he has a good car, and is quite a keen driver - that's 3,410km by road... So - driving 15 hours a day, that's an 'averaged' 75km/h. Allowing for towns and other built-up areas, that's do-able - but you wouldn't be doing any sight-seeing...
Visitors / Tourists might be better to allow 8-10 days, to get a look at some of the more interesting regions, if doing the Melbourne-Perth trip. If doing Sydney-Perth - that's 3,910km - allow at least another day, preferably two.
Sydney-Melbourne by road nowadays is about 880km. Which is rather shorter than the 960km it used to be when in the 1970s-'80s, for 16 or so years, I did the Sydney-Melbourne-Sydney round-trip 'Milk-Run' 3 times a week by semi-trailer. Since then, the Hume Highway has been routed around some hilly areas, a lot of towns and built-up areas bypassed, so is now an easier drive.
While many regions are more "modernised" now - there are still plenty of places where you can drive all day without seeing another vehicle, or where it's 400-500km between water and fuel stops. Google "Gunbarrel Highway" and have a look at the photos.
Folk from the EU and US sometimes assume that "highway" means a sealed road with water and fuel stops at least every few tens of kms. In Oz - closer to the coast, it can mean that. Inland in the Outback - "highway" often means a dirt road, graded once a year - maybe. And the water/fuel stops can be hundreds of kms apart.
City folk from here - or Tourists - not really understanding what to expect - can still "do a perish" in the outback. That is, their vehicle breaks down, and they die of thirst. When traversing a long "open region", travellers are supposed to "notify intentions", including ETA at destination - at the last Police Post before entering that area. Then, once they're a couple of days overdue - a search can be started.
So, you don't only need water (I've just noticed I'm putting "water" before "fuel" in the comments above - beause that's how you should "think it") - for the planned duration of such trips - you need enough to cover all persons in the party, for a search period. Don't forget the properly formulated salt tablets - you lose a lot of water per day, under those conditions.
Temperatures can be well over 45C in the daytime, and near freezing at night, for long periods. Estimate the amount of water needed per person - then triple it. And add "three days worth" more, in case the party had to sit-out a search period. Take some shade-cloth and cording. Once the engine stops - so does any air-conditioning. Daytimes, you cannot stay inside a parked vehicle.
The Outback is a wonderful, and can be incredibly beautiful, place - and nobody - Oz or Visitor - should miss seeing at least some of it. Picking the right Season for a Deep outback trip is important...
And never forget - the Desert Mother watches all - She has no favourites - and the Foolish and/or Careless - She will take into Her embrace - Forever...