There's usually something on every Thursday (Wil Anderson and a bunch of other comedians were there this week). Some nice bands usually set up as well. Other than that, the bar(s) are open every weekday and it's up to you to make something social if there aren't events.
Also we have a beginning, middle and end of session party every semester. We also have Foundation Day, O-Week (the best of all NSW universities), Oktoberfest, maybe one or two other fairly large events.