Off-road in OZ
to the HUBB!!
Riding around OZ sounds like a great summer. But up north you might run trouble with the wet season. I would target your ride to do as much of Southern Australia where it will be slightly cooler and less hassle with flooding, especially if you travel during Dec/Jan period.
Later in the year, ie from March, then head north and get your fill of Kakadu, Kimberly's, Pilbra, Bungle Bungles etc
As for dirt/off-road you will inevitably need topographic maps because there are very few GPS units availabe with only off-road and as far as I know they don't show when a road is tarmac or dirt, but I would happily stand corrected if anyone knows which GPS shows the difference.
In my experience, stick to the Great Dividing range on the east coast and away from cities and towns and you'll usually find a dirt road somewhere but the reall dirt (read bulldust, sand, mud and gravel) will be in the outback and WA.
You should also try ADVrider forum for people who have mapped off-road tracks.
Garmin 60csx for touring Australia - ADVrider
Garmin topo Australia ?? - ADVrider
Failing that there are 101 books on 4WD tracks around Oz, try a local bookshop you should find you're off-road heaven (or hell) there.