We can't really help you in this area. Having travelled around New Zealand extensively with all three simcards in different devices, there was not a single company that stood out as being the best performer. For your SIMcard to work in your phone, you need to ensure that it is unlocked from your current provider prior to arrival in New Zealand. You also need to make sure that the frequency is 900MHZ or above. (any latest model mobile phone will have this). All three companies claim that they have 97% coverage in the populated areas of the country. Naturally on islolated beaches, middle of a forest and in mountain ranges where no or very few people live you will find the reception very limited or non existent. In some smaller townships you might notice that on one corner you have reception and another corner you don't. Just relax, you are on holiday!
As a rough guide you, could do all of this with 50MB of data using your mobile phone:
Browse 1000 Facebook pages+ check 500 text only emails+ visit 200 webpages+ send 240 tweets+ watch 12 minutes of youtube clips.
Please note that the above figures will vary depending on download speeds and should only be used as an indication .NewZealand SIM card Shop does not take any responsibility for the accuracy of this information. The information has been taken from the telecom mobile dataguide.(6/8/2011)