AUD$100 total bill or each?
You will be hard pressed (read impossible) to have a "Fine" Dinner (evening meal) of entre (appetizer), Main Course and Dessert in the CBD for two people within that budget (if $100 is the total)...especially if you want a bottle of wine or cocktails with that...
Sydney is an incredibly expensive place to enjoy "Fine Dining"...
Nice "destination" lunches would seem to be the go...
There is also a nice dinner cruise on Sydney HarBour that leaves from Circular Quay...from memory it would fit that kind of budget...
Having said that, once you get away from the waters edge or dispense with a view there are heaps of places that are "fun" and off the beaten track too...Belgian Beer Cafe (
http://www.belgian-beer-cafe.com.au) is one such "fun" place in the Rocks area that I've taken people to before...not real romantic but it is a fun atmosphere and the food was top notch...