The South Island is sometimes called the "Mainland". While it has a 33% larger landmass than the North Island, only 24% of New Zealand's 4.4 million inhabitants live in the South Island. In the early stages of European (Pākehā) settlement of the country, the South Island had the majority of the European population and wealth due to the 1860s gold rushes. The North Island population overtook the South in the early 20th century, with 56% of the population living in the North in 1911, and the drift north of people and businesses continued throughout the century.
Link to source: South Island New Zealand