Another vote for Next G network + phones. I switched from Optus prepaid to Telstra prepaid, and believe it or not, T are actually cheaper/more bang-for-buck when you factor in their "Talk + Text" bonus (compared to the Optus Turbocharge prepaid I was on before).
I cannot imagine flying up here in the NT without a Next G phone. My only complaint is there are simply not enough phone models to choose from if you go prepaid. I went with the Nokia 6120 - not a "blue tick" phone but from all the research I did, the 6120 outperforms many of the "blue tick" phones (this blue tick thing seems to be a bit of a marketing exercise).