1 month holiday in Brisbane/Sydney/Melbourne. Which SIM shall I get?!

Hello my antipodean friends!

My wife and I are going on a fly/drive type holiday to Eastern Australia in January. I will have my unlocked Galaxy Nexus with me and will be hoping to stick a local sim card in there so that we can book stuff on the fly, use maps to get around etc.

If anyone can recommend the best networks tariff as close to an unlimited data tariff as possible, that would be great! I will no doubt use it a little to call as well, as we will need to book camp sites etc over the phone, though I would have thought 200 minutes would more than cover us, and 100 would probably do...

I've had a look over the web but you can't always tell how network coverage will be, etc. Any tips greatly appreciated, thanks!