I've created a comparison table of our 3 mobile networks' prepaid plans and the good guys over at iPhonewzealand are kindly hosting it and will be putting up an article soon about it.
You can download the PDF table from their website:
http://www.iphonewzealand.co.nz/2009/all/prepay-comparison-table/
The table reflects available discounts where possible, so for XT, there are 2 columns which take into account the topup bonuses on that network. And there are also 2 columns for 2 Degrees, because not everyone is going to be able to take full advantage of the "Magic" pricing of 22c to 022 mobiles and landlines. There is quite a heavy requirement of a $20 topup every 30 days to always enjoy these rates, so anyone using less than $240 per year on their mobile will only have partial access to these discounts (Magic rates then only apply within 30 days after a topup).
Comments or corrections welcome. I have deliberately refrained from highlighting which network is "better" for each particular attribute (eg. calls/txts/data) because the networks are so different in their offers (great!) and individual circumstances will dictate which plan is best for you.