Reply maybe a bit late......but....with regard to South Australia, you are looking at a day lost traveling to and a day lost traveling from Sydney (2 flights, Sydney - Port Lincoln).
There are two primary operators, Rodney Fox and Calypso Star. last I heard Calypso were doing day trips but you would need to check with them if that still is the case, otherwise I believe the minimum are 3 or 4 days trips based on time of year?
http://www.calypsostarcharter.com.au/
http://www.rodneyfox.com.au/
Assuming your not after wrecks, the Sydney shore diving is light years ahead of the boat diving, this is Clifton Garden's in the harbour, http://vimeo.com/6831427 thats footage from 2 dives!! (above average viz) and as ATJ said anywhere around Kurnell is great, especially for the Weedy's. If you could hook up with anyone from St George Underwater (there is a shop of a similar name as the club), you would have a great stay.
Weather is bad in Sydney this week and set to remain for next week...hope its better by the time you get here!
Cheers,
Aengus