Why does the eastern part of south island of new zealand receive little rainfall?

The eastern part of New Zealand's South Island receives little rainfall primarily due to the presence of the Southern Alps, which create a rain shadow effect. As moist air from the Tasman Sea rises over the mountains, it cools and loses moisture in the form of precipitation on the western slopes. By the time the air descends on the eastern side, it is dry, resulting in significantly lower rainfall in that region. This climatic phenomenon contributes to the eastern South Island's semi-arid conditions.