I'd suggest going further than Mt Gambier on the first day. Accommodation is available at Portland, Port Fairy, Warrnambool, Port Campbell and Apollo Bay. The really scenic parts of the GOR start just before Port Campbell - but many of the good sights require you to stop and walk to see them.

Do you need to drive Melbourne-Canberra specifically? It's not an exciting drive and frankly I'd fly straight to SYD. Canberra is not on the main MEL-SYD highway and will require a diversion that will take you even longer. I'd spend more time on the GOR, personally and spend a bit of time walking to see everything.

Also consider the Otway Fly or even Cape Otway. Much nicer than CBR!

Audrey