It so happens that I have made a trip to NZ with an AF 80-200/2.8D ED before. I used it to get close-ups of the helis at the Twizel/Mt. Cook area where there are heli tours. I also remember using it for peacocks, deer, sheep etc. at wildlife sanctuaries, dolphins and whales around Kaikoura, the yellow-eyed penguins at Dunedin, seals around various part of the South Island, waterfalls at Milford Sound, getting a tigher photo of mountains, flowers, monuments ... generally anything where I wanted more details but couldn't cross over. NZ has plenty of big open spaces and the details sometime get lost in a wide angle. I'd bring the lens again.
If you want to save a little money and weight, there's always the
AF 180/2.8D IF-ED at around $1,200 and 760g. It's a lovely little lens. I picked up one many years ago and find it lovely for portraits as well.
If you just want nice photos without carrying a lot of lenses, I highly recommend getting the many beautiful postcards at each town.

Saves you a lot of effort and waiting... and your 24-120 would be fine unless you are very particular (in which case, you would have prime lenses already ha ha)
Shooting on a Jet Boat, I'd suggest a small (single) handheld digicam in a clear plastic bag with a hole for the lens. The boat moves fast, is bumpy, water spray is cold and you're going to have to hang on during the 360 turns. Aside from hanging on and the big grin on your face, you'll probably find any other activity to be quite challenging. Besides, I think they take a photo of the Jet Boat around the jetty that you can buy later on. Saves a bit of hassle.