For my younger sisters 18th Birthday, we booked tickets from Magistic Cruises and had a wonderful time in the Cruises, having fun and enjoying the scenic beauty of Sydney Harbour and monuments around. I personally recommend to book the "Magistic Dinner Package - Shared Party Option" from the link provided. I truly believe that will be worth for the amount spend.
Your question took me back to the good time we had together, now her marriage is coming up and now I am busy organizing and coordinating it. Big brother is always busy by the way..
If cost is the concern then I advise you to buy food from the locations that you wish and move out to some private locations where its not busy (say beach side ) with your dear ones and enjoy your time there organizing your time there as your wish.. Hope that would be a great time worth remembering organizing and moving there :)
Or else you can also refer www,bestvenues.com.au for choosing your Dinner venue.
Personally I recommend you to do something different so as to make your big day worth remembering.
So Happy Birthday in advanced, LoZ !!!