I just got back from a trip around the South Island of NZ - my situation is a little different, Vodafone were running a campaign where customers from Aus could use their phones as they normally would back home, for no extra cost until 31.1.17.
However, from what I saw, it looks like SparkNZ could be a good option? I didn't see prices, but might be worth looking in to that :)
Another thing to keep in mind - Wifi access is pretty good in New Zealand, but if you need a sim card, take a look at Spark.
Milly about 4 years ago
I'm from US normally using Verizon. Bought a SIM from Vodaphone in AUK airport for $49 with 100min talk, 200 texts and 3gb data. I added 1 gb after ten days for the last two days for add'l $20. I used phone for google maps, researching sites, making reservations, hot spotting my computer, posting social media, getting pirated by my teenage sons in the RV. FYI hold onto the card until photos taken off. over 3 years ago