Although not entirely vegetarian, Otto in Woolloomooloo have a huge range for vegetarians and vegans. Same with Berta in the city. I've been to both and accidentally eaten an entire meal without even noticing we hadn't ordered any meat products.
Hare Krishna restaurants are usually very good and always pure veg...no meat, fish or eggs. Although they're not strictly vegan there are always heaps of vegan options as well. Govinda's in Darlinghurst is great (112 Darlinghurst Road) as is Hari's which is located at 157 Broadway, Ultimo.