I don't normally like talking about mobile phone plans because they are poorly marketed in Australia.
I personally use TPG - their $18/month plan works well for me, and I also have 2 of their $1/month SIMs for my iPads. I rarely pay more than $1/month, and when I do I'm ok about it. The plan is run over Optus's 3G network.
Kogan has just released a package based on Telstra's 3G 850 MHz network. It seems to be quite specific to that frequency, so if you're interested make sure your phone works with that frequency.
For $299/year you get unlimited Australian calls, sms, mms and 6GB of data per month. Not bad.