A single-phase simulation model was applied to the hot-water hydrothermal field at Wairakei, New Zealand. A two-dimensional areal analysis was made of the production aquifer under steady state and transient flow conditions, allowing vertical flow of heat and fluid through an overlying confining bed. Calculated temperature and pressure patterns correlate well with observed patterns until approximately 1963, when increasing quantities of steam in the production aquifer invalidated the assumption of single-phase flow. For further simulation of the Wairakei reservoir the numerical model will need to be extended to incorporate phase change and three-dimensional flow. Preliminary results, however, indicate that the response of hot-water hydrothermal systems to exploitation can be simulated by using a mathematical reservoir model based on a Galerkin-finite element approach.