I don't know what Australia's laws are on the issue. I know that, here in Canada, cyclists legally have to ride on the road, as they are considered to be a vehicle, and while I would assume the same holds true down under, I don't know. That being said, provided the law doesn't forbid it, definitely the road. Granted, there's a slight danger of being hit by a vehicle, but if you respect the rules of the road, and pay attention to the traffic around you, there will be very few problems.
Riding on the road has always struck me as being more of a thrill. I'm not sure if I actually go faster, but it seems that way, especially since the ride is considerably smoother. It really comes down to personal preference, I suppose, barring any legal issues. If you feel safe enough on the roads, ride on the road.