Hey NMC,
Here're a few good things to take:
Water, SAR planning, HF/CB radio, and music have all been mentioned. You may also find useful: First aid kit, Food, Sleeping bag, and EPIRB/ELT (if you don't have an HF/CB radio)
Also:
Two spare wheels (esp. if you've got a new car with one of those "razor blade" spare tyres), tyre pump, jump leads, engine oil, radiator fluid, some of that crap that you buy from Wynns that plugs radiator leaks, emergency windscreen (a plastic sheet that costs about $10.00 from an NRMA shop).
As mentioned, driving outback at night is a good way to get "up close and personal" with plenty of Aussie wildlife. Dawn and Dusk are also very active kangaroo times (they love headlights). Not sue how far out of Sydney you have to worry about wildlife, but at a guess, I'd say when the houses stop.
Best advice (My advice) for road trains is to get in front of them, and stay in front. They take a lot of getting around, so give yourself plenty of room. Have a "go - no go" point so that (should on-coming traffic suddenly appear) you either commit to passing, or slow down to get in behind.
Once you've passed them, put your foot to the floor so that you're at least 1 km in front of them before resuming your earlier speed. (They take a lot of stopping and you want that buffer zone).
If a Road train is going faster than you (and they may) give one long flash of your headlights when the last bit of the road train has passed the front of your car. That way the driver knows he has passed you and can move back into the left lane.
Hope this helps
DIVOSH!