The Vice-Chancellor's Book Fair at the University of Sydney is an annual event for all the book lovers out there wanting a bargain read. It's held in the Great Hall, which makes for absolutely gorgeous Harry Potteresque photos. The books are separated by genre, encompassing everything from young adult fiction to military history and antiquarian books. Take a look at the website and ask the people working there about the special days where you can fill a whole box of books for $10 or so - well worth a second trip if you're looking to fill your library.
It's on 10am-4pm but can get busy during lunchtime periods when the students wander over, so try to avoid the main times. Don't forget to bring your recyclable bag to carry all your books home, unless you prefer a box (bit ungainly for buses and public transport). Oh, and bring cash if possible.