Well depending how much you like hills Tassie is not the best destination.
What season are you going?
Maybe in Queensland from Cairns - Byron Bay. It is 1800km according to google maps and I would imagine it would be a pretty nice ride and reasonably flat almost 8 years ago
There is a great webpage which describes numerous sections of the around Australia bike ride.
Pick you section.....and head off. Just stay away from Qld/NT during the wet (Oct-April) season and you will be fine.
Don't forget the UV. almost 8 years ago