Hi - My parents came over to visit last year and asked exactly the same question. We tried various travel agents but none of the tours started from Adelaide - they all started from Melbourne. We ended up doing a road trip - Robe, Narracoorte, Mt. Gambier then the Gt. Ocean Road, ending at Torquay then driving back through Ballarat and Horsham.
From Mt. Gambier we drove south and along the coast through Nelson to Portland. This seemed to be endless forest and whilst very pretty, I wouldn't do it again. From Portland we drove to Port Fairy, through Warnambool, Port Campbell National Park and on to the 12 Apostles. Then on through Apollo Bay, Lorne and ended up just outside Torquay.
The road from the 12 Apostles to Torquay is very slow going, as it follows the coastline. Maximum speeds of 30-40kpm as the road is very twisty. Having said that, it was the most spectacular scenery and well worth the journey.
If we had to do it again I think we would start from Melbourne, drive along to the 12 Apostles (perhaps to Port Fairy) and then drive back to Melbourne (not all in the one day I hasten to add). The rest of the journey along the coast back towards South Australia is just trees, trees and more trees as far as I remember.
Hope this helps.
WEndy