The Waiotapu geothermal system occupies the central part of the Taupo Volcanic Zone (TVZ). With a surface area of 17 km(2) and a natural heat discharge rate of some 550 MW, it is one of the largest in New Zealand. Between 1957 and 1962 seven wells were drilled to a maximum depth of about 1000 m. The highest temperature measured in these wells was 295 degrees C. In contrast to most other geothermal systems of the TVZ, the rising plume of hot water shows a pronounced lateral component due to the position of the system on the flanks of a hydrological high. The presence of thermal features generally associated with rising vapours, such as fumaroles, mud peals and acid sulphate springs, suggests that the major upflow of hot water occurs over the northern sector of the field, close to two rhyo-dacite domes. The magma bodies associated with these domes may represent the heat sources for the system. Neutral CI waters are discharged some 4 ion to the south from a series of boiling springs and a large, sub-circular pool occupying a hydrothermal explosion crater (Champagne Pool). The chemical and isotopic compositions of Champagne Pool water reflect extensive non-equilibrium evaporation of a deep water with delta(2)H = -40 parts per thousand and delta(18)O = -2.5 parts per thousand, in a process similar to that governing evaporation from steam-heated pools. The CI content of the parent water is 1250 mg/kg, its CO2 content is, at about 0.1 mmol/mol or 240 mg/kg, very low. The S-34 content of H2S corresponds to +5.3+/-1.0 parts per thousand, and the C-13 content of CO2 to -7.3+/-1.2 parts per thousand. Geochemical evidence suggests that the Waiotapu system is linked hydrologically to its neighbouring systems Reporoa and Waikite. Each of these, however, is likely to receive additional input of heat and chemicals from separate sources.