Grant says the major reason for caps in this country is lack of capacity between NZ and the rest of the world. If you are in the rest of the world, especially in countries like the USA or UK, a large amount of their data is transferred from within that counrtry, keeping speeds up and costs down so they don't need to put data caps on.
Almost all of our data has to come in through our connections to other countries, which causes congestion and makes costs high. Data caps are a way of limiting data downloaded by large users which keeps the price down for lower users.
Once we get more fibre connections to the rest of the world, a very expensive thing to do, data caps will eventually disappear because of the increase in capacity.
We are a small country with few people to pay for the infrastructure, hence the high costs. We also have very difficult terrain to get the data through.