When westerly winds rush across the ocean to New Zealand, they meet the alps and rise up over them. The air gets colder when it gets higher, and the cold makes moisture in the air form raindrops.
Explanation:
When the humid westerly airstreams from the Tasman Sea reach the West Coast, they rise over the mountains and cool. Water vapour condenses as rain, which drops on the coast and the Southern Alps. Once over the mountains, the air descends and warms up, and the moisture evaporates. hope it helps:)
