Duster I will let someone near Sydney answer most of your questions as I am just over 600m north. However, our roads are quite reasonable but not interstate as you have. There would be no need for offroad excursions unless you wanted to. A GS1200 would be fine for either, but a tourer will also as you will have a better setup for pillion plus usually panniers.
I have a couple of online mates coming over during 2007, most are also heading out of Sydney. You will enjoy the trip.
Personally I would go with a big tourer as thats what I am used to, these guys seem to have been around a while.http://www.bikescape.com.au/rentals/fjr1300/fjr1300.htm