I don't know if things have changed, but I flew with a Riot Disco in 2002 on the way back from NZ on United and Air New Zealand with no problems. No bag required either, just seal the cockpit with cardboard. They didn't even charge me for the boat until I arrived in our fair country at which point I had a serious negotiating session with United ($75 charge to fly it from LA to Hartford). Also, I am pretty sure that some flights from Auckland to Christchurch are on larger jets. My general impression was that the kiwis were much more willing to work with you than their American counterparts. I am sure this wouldn't be the first creeker that has been taken on a plane to the S. Island.
But, if you don't want a hassle, you can usually find a creeker for cheap once in country.
"A river is more than an amenity, it is a treasure."
-Oliver Wendell Holmes writing for the majority in New Jersey v. New York