Tasman district
Absolutely spectacular. Bit of a cliche, but it's the best description possible for this activity. My wife and I went in a group of 10 kayakers (tandem kayaks, so everyone could pair up) with Kaiteriteri Kayaks in December. The weather hadn't been great for days previous to the trip, but it was excellent on the day. This was particularly worrying for me, as I'm not a strong swimmer by any stretch, however it only enhanced the sense of adventure, though. We rowed from a secluded beach in the north of the park, out to several islands (stopping for a picnic on one), and across the uncharacteristically calm "Mad Mile" to get to the Pinnacle Island seal colony. Getting up so close to the seals (they were happily swimming between the kayaks), having the smallest penguin I've ever seen swim right up to me for a good, long look (I think he was lost!), the beautiful scenery, and the overall feeling of at-one-with-natureness, made this one of the highlights of my trip to NZ.
The guides were extremely competent, and a great laugh. The scenery was just stunning. The food was so-so, but I guess you don't want to be stuffing your face when you're going to be in a kayak on the open ocean for 7 hours! So, forget the swimming with the boring old dolphins. This is the trip you want. Just be prepared for some arm aches the next day!