Sounds like a grand plan. I wouldn’t go for anything less than a month though. If you like the coastal/beach thing, then as has been suggested Melb to Bris is a good idea. Bris to Cairns is also very nice but is a loooooong way. For something different, Darwin to Adelaide is also very popular, taking in Uluru etc.
Internal flights are relatively cheap with Jetstar (owned by qantas) and Virgin blue. Beware though as these 2 fly to some airports that aren’t at cities (eg Prestwick vs Glasgow) and are often late. If you’re short on time, fly inbetween, but if you have more time, it’s worth the drive between cities. There are loads of things to see if you’re prepared to go a bit off the beaten track.
Wicked campers are also in Australia, and will certainly be a cheaper option than most.
When you’re a bit closer to the time and have a better idea of what you want to see, send me an email and I’ll help out with some of the finer details if you like. If you take your MTB, check out http://www.mtbdirt.com.au for the MTB community in Brisbane, or http://www.farkin.net
You wont need to go to NZ as you’ll meet plenty of them in Oz 😛