Most of the country is represented in this graph. Socio-economic information about New Zealand is readily available online. Google is your friend.
These kind of stats are shown around the globe. You're onto it though- dental decay is about poverty, it's not about fluoridation in the water.
http://www.newsweek.com/fluoridation-may-not-prevent-cavities-huge-study-shows-348251
that huge study seemed to have excluded most studies from consideration