I believe I may be able to help out with this one because I recently was researching moving to New Zealand (although have decided against it for various reasons).
A vast majority of the housing in New Zealand is not insulated. As we know it is currently winter in NZ, and the country is prone to drastic weather changes and a great deal of rain. Many of the residences are very cold and very damp. Most people rely on space heaters, although they don't provide much help because of the lack of insulation. I read many stories of people saying they never experienced the cold like they did in NZ; it literally chills you to the bone. I also read many stories of people saying they had never gotten sick as much as they did while living in NZ.
All of this has led to NZ having one of the highest rates of asthma in the world. Approximately 15% of the adult population and 20% of children under 15 have asthma. For those who don't, there is still the exposure to the cold and damp environment. There is currently a push by the NZ government to get citizens to start updating and insulating housing, although the effort will likely take awhile.
I hope this helps a little. I have a strong feeling that environment plays a huge role in the problems in NZ.