Chris, Dimitri, The NSAS is probably what you're looking for ...
http://nsas.org.au
As for observing sites - the NSAS has used the carpark of the North Turramurra Golf Club which Ive tried (about as good as it gets in Sydney) and they also have another site.
There is an oval on Koola Ave Killara with I've used, find a spot where the trees shade you from the one or two streetlights that offend. The downside with this site is you have to carry your gear onto the field as you cannot drive onto it for obvious reasons.
The alternative is to head north to Mt White or thereabouts, and find a road into the bush somewhere away from traffic, or make friends with a local who has a suitable block (a case of beer might help).
If you're going as far as Peats Ridge or CentralMangrove (both possible if you know a landowner) you may as well go to the Pony Club (you'll see that mentioned elsewhere on this site).