Tetsuya's is always good for a high quality food experience in asian-inspired surroundings. A recent lunch there for six was a wonderful meal. The wine matchings were particularly good. For a relaxing (but luxurious) lunch try the Hyatt :
http://harbourkitchen.com.au/
A table by the window is typical Sydney and the weekday lunch (at $39, incl. wine) is a bargain in a 5* hotel. I can also cope with crab cakes and a beer in the bar!