There are 2 main mobile operators in New Zealand - Vodafone and ?. Both offer the usual services including Pay As You Go SIM cards, but only Vodaphone uses the standard GSM mobiles and SIM cards, so I focus on Vodafone.

The best way to get a mobile in NZ is probably to bring your own and purchase a SIM card when you get there. You can use any GSM 900 mobile phone (or dual / tri band) phones. Older Vodafone / O2 phones from the UK are GSM 900, while pretty much every recent phone is at least dual band. Just check your phone is unlocked before you leave the UK. New phones are quite often expensive and have limited availability in NZ - while second hand handsets are dirt cheap in the UK right now. Try Retrofone instead of eBay if you are after a phone quick - especially the Motorola L7089, which is the cheapest tri band phone I know of. Its not a great phone, but its £25 and works worldwide. If you are not ending up in the US, dual band will be fine. If you are in NZ and looking for a cheap phone, your best bet is trademe.co.nz - the New Zealand version of eBay. Theres lots of hansets for sale in the Auckland area, which often allow you to collect for cash.

Vodafone SIM cards cost around $40 with $5 credit on them. There are 2 possible plans, of which I reccomend Motormouth, as it allows you to call the UK / Ireland / Oz cheaply:

		VF to VF & landlines in NZ	Other NZ mobiles	Australia, Canada, Ireland, UK & USA	Rest of the world
Motormouth Prepay		$0.49 Anytime	$1.39 Anytime	$0.49 Anytime	$1.39 Anytim

Assuming that most of your mates are on Vodaphone, $0.49 per minute for all calls is pretty good. Being able to call UK landlines at any time for $0.49 per min is worth having. Calling cards and skype are both much cheaper, but less convenient.
Theres also a full range of pay monthly contract options, but I suggest you go PAYG and see if a contract would be better - but be aware that its usually a 1 year contract, which is not ideal if you are planning to stay for 6 months, as you will be charged to get out early.

Texting is popular, costing $0.20 for NZ mobiles and $0.30 for international texts.

3G and GPRS are available from Vodafone as well.