Phone calls in New Zealand are not cheap. Unlike in America, where cell phones with unlimited night minutes and free national calls are ubiquitous, you pay for every minute in NZ. This applies to both phone cards, which I used to call internationally, and mobile phones, which I used for domestic calls and the occasional international call when I could not find a landline.
There are a plethora of choices greeting the weary traveller in the international airport who just wants to call home and tell her family she arrived safely. Asking about phone cards and having a stack of pamphlets forced at you can be overwhelming; I just gave up thumbing through them and asked the clerk which one had the best rates to the US. He recommended GoTalk, which offered 2.7cent/minute calls if calling from a landline to the local access number. This is where all phone cards in New Zealand get your money, because it is often not possible to use a landline or call a local number if you are not in one of the major metropolitan areas. I cannot find or remember the exact surcharges, but calling the nationally-available number (0800 or the like) from a payphone attracts a surcharge of 40-50c a minute (which adds up very quickly!). Surcharges of about 45c per minute also apply when calling from a mobile.
If you are planning on staying in many BBH hostels at all, the BBH phone card (offered by TelstraClear) will definitely be the most useful phone card of all. Besides the fact that your BBH membership comes with $20 of free phone calls, all of the member hostels make it very easy to access. Most phone cards only come with published local numbers for the major metro areas like Christchurch, Queenstown, Auckland, etc. The BBH card also comes with this same, generic list; however, even the most remote hostels (the Punakaiki Beach House, for one) post the local access number for the BBH card next to the phone so you won't attract the national-call surcharge. On top of this, many hostels offer free landlines, and those that do not often have a special button to press on the pay phone that bypasses the paying mechanism and therefore avoids the other surcharge. The only downside is that most hostels only have one of these phones, so you might have to wait a while/conduct your conversation in a public area, but at least it's cheap!
There are two major mobile phone companies in New Zealand: Vodafone and Telecom. I bought a Vodafone prepaid SIM card for my unlocked quad-band phone in the Auckland airport for $30 (which was much more expensive than I expected, given that a prepaid SIM in Australia costs only $10). On top of that, I bought $20 of credit so I could actually use it once it was activated.
The main reason I bought this SIM card was as an emergency backup plan. If I got stuck somewhere and had to get in touch with the outside world, it was possible; this certainly put my parents' minds at ease for when I was in the more remote areas (although I didn't actually have mobile reception there anyway!). It certainly wasn't something to be used for calling people daily, since the rates are through the roof--I believe it was around 80c a minute just to call other Vodafone members! I did use it quite often to call free national numbers (the equivalent of 1800 in the US), but as I mentioned in the above section, if you are calling your phone card, this attracts a surcharge. I mainly used it to TXT message other travellers (which was much, much cheaper than calling) within NZ and call internationally when I didn't have access to any other phone. Still, I found myself topping up my credit quite often.
I was very lucky to run across the Super Buzz phonecard in an internet cafe in Nelson. Apparently, this is sold in over 7,500 retailers all over NZ--so essentially, every convenience store that sells phone cards. This phone card allows users to call from Vodafone mobiles with no charge to their mobile account. The rates aren't as low as some other cards, but the added convenience of being able to use your mobile more than makes up for this extra expense. You can call specific countries like Australia, the UK, and Korea for 7.5c with increasing rates to other countries (the US is 50c per minute, for example). This deal is ONLY available to Vodafone customers.