Dinner for 18 in North Sydney
Sea treasure recently got pinged to the tune of $10,000 by the NSW food authority on hygiene issues. I don't know if i'd want to go there. If you're after chinese perhaps yings would be a better option.
Garfish in both Kirribilli and crows nest could be suitable. Consider also somewhere like Pino's in crows nest - hardly haute cuisine, but cheap and cheerful Italian with an upstairs area for larger groups.
For a family shindig you might also consider going to somewhere like the oaks and grabbing yourself a section of the outdoor area and dyi bbq or alternately the pizza place in there (if it still exists?). The commodore hotel also has a nice eating area (although I haven't eaten there in years).
A bit pricier but certainly value for money used to be Milsons - believe that is still of a pretty good standard...
Ummmm... or you could try the Burlington which has thumbs up in both AGT and SMh food guides in past month.
Note: North Sydney (in fact the north shore) isn't known for its interesting culinary locale!