In prose, the two main islands of New Zealand are called the North Island and the South Island, with the definite article. It is also normal to use the preposition in rather than on, for example "Christchurch is in the South Island", "my mother lives in the South Island".Guardian and Observer style guide: N ("New Zealand"), The Guardian. Retrieved 6 May 2012 Maps, headings, tables, and adjectival expressions use South Island without "the".
