different states different laws.
Qld states 250cc only, no bigger.
others will let you have some crazy bikes on your l's
may I suggest that if your doing 12k+ (is that each way or return?) on the road to maybe look at a motard as more of your k's will be on road then off road. I have a motard on basically full slicks and it will still handle the dirt, not as well as knobbies but I will absolutely kill any bike with knobbys on the road.
And if you are wanting to do more dual purpose road / dirt look at tyres like the Avon Distanza, good tyre both on and off road.