there is a huge peice of beautiful bushland between sydney and the central coast (north of sydney) with a river following all the way up (called the hawksbury river). Its so wild and sucluded that nobody could (nor would) both you if you were to set up camp there.
just a bit south of newcastle, right? that river's beautiful.
for anyone else interested, i'd also suggest west of sydney, toward the blue mountains. there's a coal route that runs from newcastle down to sydney and then west, with enough small yards or sidings to catch a ride before it runs out to a mine. (that is, if yer riding empties out.) loaded coal headed east is most likely going to the port in newcastle, and secondly to a port just southeast of sydney. you can also very easily catch a commuter from sydney central out for free, as far as lithgow or even a few stops farther. the temps. a bit colder up in the higher elevations, so the time of year is probably important.
i would say the climate out at alice springs would be a bit too harsh for wilderness camping, with extreme heats during the summer. that said, i didn't make it up that far while i was there last year, as i found hitching in SA to be pretty difficult at that time of year as there were hardly any tourists.
the southern highway west of melbourne, starting at about warrnambool i believe, runs along the coast and is fucking gorgeous!! there are koalas and wild king parrots that will land on yer arms if you have food for them. (the aboriginal tribe that lived in the area before white settlers befriended them for thousands of years...) the route isnt widely settled, and i'm sure you could find an awesome spot to set up camp while still being able to catch a ride to a town for food or whatever else if needed...