If you want large cheap meals, go to any of the thai restaurants on King St - about a 5 minute walk from uni. They do $6.50 thai. You can generally make it in an hour lunch break, but sometimes it can take a while for the food to arrive. You will find lots of students at these restaurants!
Otherwise the main food outlets are located in Wentworth building or Manning. These have food court style eating areas.
Saying all that, I'm trying to bring food from home most days - saves a lot of money.