Check out the data from the 2013 Census among kiwis:
In just 15 years, those claiming to be affiliated with Christian religions have dropped from 60% to just under 49%. The only redeeming news is that those in the “evangelical, born again, and fundamental” category actually have risen by a factor of 11.2 percent. So maybe the upshot here is that mainline religions are losing their luster while evangelical Christianity is rising?
The NZ reality is more nuanced than you suggest.
See this news article http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=11196786
One key factor is migration, with people of faith coming to live in NZ. This is one factor in growth in Catholicism.
Amongst NZ Europeans years ago many people would list Anglican as their religion like “To be Thai is to be Buddhist”. But now they list “no relgion” as their nominal ties to the church have been cut.
( see http://mikecrudge.com/2013/07/19/pop-quiz-when-did-regular-church-attendance-peak-in-new-zealand/ )