I live in Sydney and have dived up and down the eastern Australian coast. I do a lot of diving in Sydney, but on the most part wouldn't rate it as world classs. Yes you can have some good dives, but I wouldn't travel half way around the world for it. I personally like the southern dive sites out of Cronulla, but they're all much the say. Some of the deeper wrecks can be fun also if you're into that.
My suggestion would be drive/dive from Sydney to Brisbane. My picks would be South West Rocks (5 hours north of Sydney), North Solitary Island/Mullaway (1.5 hours past that) and Byron bay (visit Nimbon too and buy some cookies).
If you're going to the GBR get on a good liveaboard and get to the Coral Sea. Day trips suck and are full of gumby snorkeller whom seem to take over boats. The ribbon reefs can be great, but are second rate to the coral sea.