On the basis of chemical composition, the waters of hot springs of the Waio-tapu thermal area have been divided into four classes: acid sulphate waters, sulphate-chloride waters, chloride waters, and bicarbonate-chloride waters. The origin of the waters and the relationship of the hot springs to geological features is discussed. The natural heat flow of the Waiotapu thermal area has been estimated as 302,000 Kcal/sec above 15°C.