I just came across a web company that books discounted tours for popular activities in New Zealand (and other places). I found some favorable reviews on TripAdvisor from folks who have used it.

The savings don't seem huge, but you might be able to save a few bucks here and there.

www.viator.com

I have had extremely good results from Viator. It is especially nice to have a taxi waiting for you to take you from airport to hotel in an unfamiliar country. Fare is paid ahead so no worries about exchanging money right away. One tour I have done has been great.

Just a follow up...

I used Viator to book my Heli Hike tour in Franz Josef. Saved some $$, and Helicopter Line accepted it without any trouble. Also, I no foreign transaction fee on my credit card statement!

While we were in NZ, we also discovered Rankers, which offers a bunch of 10% off rates with a card (http://rankers.co.nz/more/card_participants), but since we booked most of the tours prior before we left the states, we didn't use it. Wish I had known about it earlier.

I've used them through asiaoz.com a few times in Asia for transfers and worked fine.

if you use capitalone credit card... they do not charge you a transaction fee!!!
So if you travel a lot this is great savings