The Big Day Out - amazing one-day festival, usually in late January. Get tickets ASAP if there are any left: http://www.bigdayout.com/
Byron Bay Blues and Roots festival is fantastic too! Easter Long weekend. Get in early for tickets, campsites etc http://www.bluesfest.com.au/ about 9 years ago
a couple of the big ones in early 2010 are
Future Music festival (Franz Ferdinand, Empire of the Sun, David Guetta are headlining) www.futuremusicfestival.com.au/,
Good Vibrations (The Killers and Basement Jaxx) www.gvf.com.au
a smaller one in darling harbour called A Green Fairyland http://www.greenfairyland.com/.
Big Day Out is probably sold out in Sydney but there might still be tickets available for the Gold Coast or Melbourne if you also wanted to take a trip at the same time :) about 9 years ago