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Development of a computer model of the Waimea groundwater system was prompted by problems of water management already being expereinced by the Nelson Regional Water Board in the water-short Waimea Plains. The aims of the modelling were to test existing concepts of the dynamics of the major aquifers, to identify areas or parameters requiring better data for management and then to evaluate the effects of possible future pumping regimes on seawater intrusion, groundwater and river levels. A quasi three-dimensional computer model of the groundwater and river system was developed, calibrated and tested. The model was based on the three-dimensional finite difference code of the U.S. Geological Survey. Modifications were made to this program to improve the representation of river-aquifer interaction, to allow rewatering of dry nodes and to improve data input and output. Thre sub-models were also developed to provide data input for the model: rainfall and irrigation recharge, river-aquifer interaction and irrigation pumpage estimation. The groundwater model successfully reproduces the spatial and time-dependent water level fluctuations in the shallow unconfined aquifers (UA), the Upper Confined Aquifer (UCA) and the Lower Confined Aquifer (LCA). It also simulates flows in the Wairoa, Wai-iti and Waimea Rivers which lose much of their flow to the aquifers during dry summers. Management simulations identified seawater intrusion, excessive water level drawdowns and minimum allowable river flows as the constraints on increased exploitation of the LCA, UCA and UA respectively. The model indicates potential seawater intrusion to the LCA under present pumping stress, while the UCA appears currently under-utilised. Increased localised yields from the UA are shown to be achievable, but an existing management objective of maintaining a minimum low flow in the Waimea River limits the area of available water to the Delta zone of this aquifer. Benefits in developing this regional groundwater model include: - the focal point it provided for the assembling and evaluation of the large amount of geohydrological data on the Waimea aquifers; - an improved conceptual model of the dynamics of the system; - a useful management tool which has tested many alternative regional water allocation strategies and consequently allowed the adoption of realistic water management policies