I’m currently working at cell phone company (as I type SHH!).
NZ uses CDMA and my company (in canada) can use the NZ network. I’m sure GSM is there as well but, hey, CDMA is better. 
The sin stuff is also protection against Subscription Fraud and other types of shit we deal with. Heck, I’m currently working on disabling an account in which someone (husband and wife) used one another’s credit infomation to open accounts (gee, I didn’t know MY sin can’t be used by my wife!).
And what has been said here is very true. The phones are several hundred dollars (even that ones that are listed as “$49”) because they’re subsidized. If you want one, you need to pass a credit check with Equifax.
According to my roaming information database, NZ uses 800 MHZ CDMA.