An island of New Zealand separated from South Island by Cook Strait. It is the smaller but more populous of the country's two principal islands. one of the two main islands of New Zealand, which includes Wellington, New Zealand's capital city South Island. Island (pop., 2001 est.: 2,849,724), New Zealand. The smaller of the country's two principal islands, it is separated from South Island by the Cook Strait. It has an area of 44,702 sq mi (115,777 sq km). A growing majority of the population of New Zealand lives on North Island, concentrated in the cities of Wellington and Auckland