The heat flow at 23 sites in the North Island of New Zealand was deduced from the geothermal gradients measured in drillholes and the thermal conductivities of the rocks penetrated. The flow is below normal to the south-east and above normal to the north-west of the active volcanic belt. Within and near this belt, zero geothermal gradients were measured, although the combined heat flow from the hydrothermal fields is known to be equivalent to a mean heat flow 20 times the normal value for the whole belt. The hydrothermal fields are regarded as narrow upflow zones in convectively circulated meteoric water systems. The zero geothermal gradients suggest that a large part of the whole volcanic belt and some areas immediately west of it provide the intake for these convective systems.