New Zealand is very much a backpacker's country; this is obvious by the sheer number of hostels throughout the country. Even in some of the most remote areas, there are often one or two hostels catering to the travelers passing through. Because there are so many hostels--most of which have an average price range of $20-25 for a dorm bed--figuring out where to stay in NZ isn't nearly as easy as in other countries where you don't have much of a selection.
There are three different hostel "loyalty programs" in New Zealand. The first, and most well-known outside the two islands, is YHA, or Youth Hostelling Association (and known as Hostelling International worldwide). There are approximately 50 hostels in NZ that carry the YHA label. Many of these are owned by the YHA, but a few are associate members that incorporate the YHA discount system. All YHAs have a $3 surcharge for any bed if you are a non-member; so, if you plan to stay in a YHA for more than 9 nights, it's worth buying the $40 card, especially since it also comes with a phone card worth $10. Also, many restaurants and activities offer special discounts to YHA members.
However, you should do your research before you buy the card; I only ended up staying in one YHA for my entire trip because I'm not a huge fan. The hostels are always clean and pleasant places to stay, but I often find them too large and impersonal. Plus, they rarely provide non-pay phones, which is a regular occurrence at BBH hostels, so calling home/making phone reservations incurs a huge (~40c a minute) surcharge. One big upside to YHAs is that they nearly always have fantastic internet facilities that are reasonably priced and sometimes have USB connections/memory card readers. This is convenient if you want to use the internet in the evening, since you don't have to leave the hostel. Just be warned that the internet is all on a pre-paid system (you buy a code for a certain number of minutes from the front desk) and if you decide you want to use the internet after reception has closed, you're out of luck!
BBH (or Budget Backpacker Hostels) is the largest association of hostels in the country. Boasting nearly 370 hostels spanning from Kerikeri in the far north to Invercargill in the south, you are likely to find a BBH hostel in nearly every place you want to go (for instance, I stayed in a BBH hostel in Punakaiki on the South Island's West Coast…a town that is 50kms away from both petrol and groceries!). I bought a $45 BBH card on my first night in NZ, because it pays for itself after about eight nights. This is because the BBH card gives you a $2-3 discount per night (depending on the individual hostel's policy) and comes with a phone card initially loaded with $20. This phone card was definitely the best investment I made in New Zealand, since most BBH hostels will provide the local phone number for the phone card and either have a non-payphone or have an exception so the phone does not charge if you are dialing the BBH number. I have explained more about phone cards in another entry in this journal.
BBH hostels are only members of an association rather than hostels owned & operated by a specific group like the YHA. This means that the hostels vary widely in their upkeep and character. Most of the hostels are fantastic (although there are definitely dodgy ones as well), and they are often smaller and have a much friendlier atmosphere. The association publishes a guide to its hostels that it updates yearly; this guide is an absolute essential to any traveler in NZ. Everyone I met carried it in a convenient pocket, where it could easily be reached and pored over. This guide has all of the essentials for each hostel: its name, address, phone number, website/email, and a map, as well as more personalized content, including a short description and BBH rating. These ratings are mined from a survey filled out by everyone staying in BBH hostels on a certain night of the year (I gleefully filled mine in while staying at Wanaka Bakpaka in late February), so they aren't just an arbitrary score given by some reviewer. I found that the numbers were normally spot on, and I always knew that if I could get a booking at a hostel with a 90% or better rating that I would be in for a real treat. The BBH also provides its own website where BBH card holders can log in and contribute to "online ratings" without having to wait for the yearly survey. Often, most hostels only have 3-4 ratings online, but some of the comments can be helpful.
The last loyalty program found in New Zealand is VIP. They, at least to me, seemed much less visible than the other two associations in NZ, even though they claim to be the world's leading hostelling association. They apparently have 70 hostels in NZ, but if I stayed in a VIP-associated hostel in my ten weeks, I was not aware of it. The VIP website isn't too helpful, so I don't know how much the card costs, but it says that you can get at least $1 off at any member hostel in NZ, which is significantly less than the other two cards provide.
Overall, my recommendation for the most useful card in NZ is the BBH card, but you may have a different opinion. You don't have to stick to one card, either--buy them all if you want!