Productivity improves, and international competition often ensures that the benefit of that improvement is passed on to consumers, rather than retained by producers.
Reflect on the huge fall in the real price of computers - a fall which dwarfs the falls suffered by New Zealand land-based exporters - or the price of television sets, or of motor vehicles, or of international air travel, or of long-distance telephone calls. Large price falls are by no means unique to the products which New Zealand exports.