Most of the guided stuff is on Fridays/at the weekend if that is when you are here because its winter, although I think you can go on guided night dives with prodive on Tuesdays or Thursdays, I’ve done a couple and they are really good fun. The mcfadyen site is great for local info.
Great dives (in no particular order):
Manly – shelly beach. shallow shore dive – great for macro life (octopus, cuttlefish that sort of thing and carpet sharks). I’ve dived with both prodive manly and dive centre manly for guided shore dives at the weekend, both an easy walk from the ferry.
Other side of the harbor - camp cove, shallow shore dive, (8m max) so much macro stuff, seahorses that sort of thing - I’ve dived with DC Bondi and Prodive Coogee (guided). Both good, prefer prodive coogee, their enthusiasm for local diving is infectious. You tube link for campcove, although the worst vis ever:[video=youtube;J5UXHFnryZ8]http://www.youtube.com/watch?v=J5UXHFnryZ8&feature=plcp [/video]
If you want to see the endangered grey nurse sharks – it’s easier with boat diving, I go with prodive – either from rose bay wharf (can get there by bus or taxi, if taxi say the catalina restaurant or they can get a bit lost!) or manly wharf. Don’t forget some seasick tabs for that trip. <span lang="EN-US">[video=youtube;AKZRA2t5tJI]http://www.youtube.com/watch?v=AKZRA2t5tJI&feature=plcp [/video]
So Sydney is pretty good for diving!
I would second a trip to Byron bay - Julian rocks. It is actually my fav. place to dive in Aus, warmer than Sydney, you get a lot of tropical fish, grey nurse sharks etc, and often better vis than Sydney. I fly into Coolangatta and drive down (about 40 mins) to Brunswick heads and go from there. You tube link for typical dive:
<span lang="EN-US">[video=youtube;RYomH8GwN-Q]http://www.youtube.com/watch?v=RYomH8GwN-Q&feature=plcp[/video]
Cant comment on the GBR at the moment, but am going there this weekend and will be doing some day trips so if you are interested i can post again.
Hope that helps!