Rod, As far as I'm concerned, Port Hacking and Georges River, although both good water ways, offer parking problems for large groups. However, Narrabeen Lake or Appletree Bay have plenty of good parking areas (you do get stuck with a $7 fee at Appletree, but it's probably worth it). Sydney Harbour would possibly be OK west of the bridge parking-wise. I'm not talking 100% from experience, but Google Maps gives one a good idea of parking spaces.
Personally, I reckon Narrabeen Lake, as it's next door to Narrabeen itself, so last minute purchases of supplies is easy, it's not too bad to access from most places in Sydney, and it offers a good variety of fish and locations. But then again, the Georges, upstream of the Liverpool weir could also be good.
I'd welcome other Raiders' inputs as my recommendation is by no means going to be welcomed by at least some would be participants.