Geophysical methods were used to investigate the possibility of exploiting the Onepu hydrothermal resources in the Kawerau papermills. Changes in the hydrology and chemistry of the hot springs in the last fifty years are attributed to downcutting by the Tarawera River into a Recent alluvial fan; surface runoff has given place to seepage and alkaline waters have given place to acid. Electrical surveying extended the known hot-water zone across the Tarawera River, where test holes gave satisfactory steam and water discharges, and exploitation followed. The naturaf heat flow was estimated at about 25,000 k.cal. per sec. (above 12° C), but the potential output of eight drillholes at 100lb per sq. in. pressure is 2.7 times this figure. High enthalpy discharges and enthalpies which rise with the wellhead pressure may result from tapping the margin of the hot-water zone.