Hi, Newcastle is a great spot to dive from. Try the website for Charlestown Dive which I think concentrates on guided shore dives in and around the Newcastle area. Swansea Channel is a great dive already mentioned. Port Stephens is just north of Newcastle and a couple of dive companies operate from there, check out Prodive Nelson Bay (it might be Prodive Port Stephens - same place), Try to get out the Broughton Island.
100miles north, ScubaHaven at Port Macquarie runs a boat out to the Cod Grounds which is home to a mob of grey nurse sharks.
An hour south of Newcastle, you are on the Central Coast and you can dive on our new artificial reef the warship Adelaide. Contact Les Graham at terrigaldive.com.au. The northern waters of Sydney are also accessible from here and ScubaShack at Ettalong run a boat out to these sites.
That should keep you going for a couple of weeks!