There have been media rumors circulating around for a little while that NZ might not get the business for the next three films. This was definitively debunked and put to rest on December 15th. See this article in Variety for more details.
Also, here’s a short video of the New Zealand Herald’s coverage of the actual announcement by New Zealand’s Prime Minister, John Key, with some comments by James Cameron himself. Jon Landau of Lightstorm Entertainment and Paul Hanneman from 20th Century Fox are also in attendance: