Waikato (wĪˈkäˌtō) [key], river, 264 mi (425 km) long, rising in Lake Taupo, central North Island, New Zealand, and flowing NW into the Tasman Sea. It is New Zealand's longest river. The Waikato power plants are the main source of hydroelectricity on the North Island. Coal is mined from fields including reclaimed swamplands. Dairy farming, lamb production, and grape and kiwi growing are important activities in the basin.