My favorite bargain buffet in Sydney is "Bungarribee International Buffet", located at the Workers Club in Blacktown. They have a fantastic selection of soups, Asian dishes, roasts, salads, fried foods, and desserts. They also have many different specials on at different times of the month. It is a really great buffet to bring the kids to - they love to choose their own food, and its a great relaxed environment!
Star Buffet is pretty amazing. I have only been to the one at Seven Hills RSL but there are a few others around Sydney. They have a huge range from a roast dinner through to sushi. Dessert is just a sugar coma. It is pretty reasonable with 'all you can eat' dinner on Friday and Saturday nights just $25.80 for non-member adults.