Shouldn't DINGO excursions be open for booking soon?
I guess you could do the Great Ocean Road if you left on the first day and didn't mind getting back into Melbourne late. I don't think I'd chance it on the second day.
I think it's too much time in a car or bus for me (plus I'm looking at plenty of bus time in NZ).
Edited to add: here's a tour website which offers trips to the 12 apostles
http://www.melbournetours.com.au/
