byron bay - depending ont he time of the year, the water can be 26 degrees to about 17 (degrees C that is) a quick google will pretty much fill you in.
Re Port Douglas - Dived there twice. the reef is very pretty, clear water, lots to look at, but the boat ride is a killer. 2 hours is about right. for Port douglas, I'd suggest going with asmaller outfit, the big boats tend to be just a bit too...crowded. Haba, Poseiden and Aristocrat are some of the better dive operations out of Por tdouglas. (there are more).
As far as dive spots in aus - I hear Ningaloo is a very good choice - the a bit of a pain to get to.
Damn, I just realised, this country has 22,812 miles of coastline. So much diving to do, so little time....
Z...