Most phone company web sites include coverage maps, but these rarely include enough detail to be sure of how a given network will perform. Android app Sensorly collates data from individual users and compiles these into coverage maps to give you an on-the-ground perspective on whether you'll get signal.
Sensorly has extended its existing US service to Australia and New Zealand. Right now, Sensorly claims only to cover GSM reception, which might limit its usefulness for most 3G-hungry smartphone owners.
I'm not personally equipped with an Android phone to test this out; if you give Sensorly a go, tell us how useful (or otherwise) it is in the comments. Sensorly is a free app for Android phones only (though an iPhone version is promised in the near future).