Some more for you (the ones I mentioned before). I don't know if I'd call these guys mainstream. They are played on the radio every now and then, but I doubt anyone out of Oz would know them. Anyways, on with the groups.
tism (stands for This Is Serious Mum). No one is quite sure who these guys are, as they're always in bellaclavas (sp?). For the Americans (and anyone else) Bellaclavas = Ski Masks. Like those ones robbers always wear in bad movies. Anyways, their music generally has a (sometimes very cynical) point to it, while at the same time being funny. Their most popular song is "Greg! The Stop Sign!". They recently came out with a "best of" album, which gives you a pretty good collection of their most well known works throughout the years.
http://www.tism.com.au -- tism.
Sonic Animation. These guys are techno and may be known by other people for their recent song "I'm a DJ". For Triple J listeners, they were on one of the Hottest 100s for their song "Theophilus Thistler (An Exercise in Vowels)". I highly recommend listening to that song or at least reading the lyrics and thinking about how much of a mouthful it is to sing/rap. Anyways, on with the website.
http://www.sonicanimation.com.au/ - Sonic Animation.
Just one more that I don't know too much about. A friend of mine introduced me to the music of The GO Set. While they haven't hit big yet, I think they have the potential. One of their main instruments is a set of bagpipes, which is pretty cool. They will be doing a tour in Germany pretty soon.
http://www.thegoset.com/ - The GO Set
That's all for now, check them out!