jtan: +1 for the Optus prepaid sims.
Have always used these when we have been visiting Australia and $2 a day gave us all we ever needed while there (unlimited calling, text and 500mb data). A couple of times we were over for a music festival and data there was a little temperamental but that was likely due to the sheer number of people at the event overloading the network.. On a standard day, no issues.
Also believe that they have a 'top up' with their $2 days (Daily Plus) for another 524mb for an extra $2 if you are feeling particularly data hungry that day. When you arrive in Aus, just pop to the Optus counter at the airport (they have them in Sydney and Melbourne, but likely at the other ports too) and they'll have you set up and activated in no time. Usually takes about 5-10min after clearing customs and you should be on your way.. :)
One thing I might add, is I do not believe that tethering is available using this SIM's. It might be possible that that is an iPhone limitation, but from memory when I tried to set up a wireless hotspot using my phone with one of these SIMs the setting was unavailable. Just FYI in case you are looking for this functionality. If you have any queries, I've sent the Optus team queries in the past via. Twitter and they've been quite quick to reply. Good luck!
Another +1 for the Optus prepaid sim.
My wife and I used them at Easter last year in Sydney with no issues. I'm also reasonably sure that I did also use it to tether to my laptop. I have an Android phone.
As we are with Vodafone and calls to Aus are included within our monthly minutes, we call forwarded all calls made to our NZ numbers to the Aus numbers. I also have a cheap dual sim phone, so placed our NZ sims in that phone so as that we still received texts that were sent to our NZ number.