I had an amazing experience working and traveling in New Zealand. I don't think it was truly necessary to use an outside provider such as BUNAC- anyone could just make these arrangements on their own. They do help guide you through the process and there was an orientation upon arrival, but after that the support was pretty non-existent. You end up just figuring out your own housing, jobs, etc.
But NZ is an amazing country with wonderful people. You need to be diligent to get work, but it is well worth it and the country is so easy to get around to see it all while you are there. Save up money before you go so you have a little flexibility in case you don't get work right away. And don't be afraid to try jobs that might be out of your comfort level- it's an adventure!