I recented asked a similar question and went last weekend. I'll post my experience in this thread.
http://www.flyertalk.com/forum/austr...and-tours.html
P.S. I believe the GOR technically starts on the other side of Melbourne, so if you depart Phillip Island for the GOR, you'll have to go back through Melbourne anyway. In other words, I'd just go back that evening to Melbourne. As it's Jan, you may not be back in Melbourne till about midnight.