Changes in land use and management practices can have significant impacts on freshwater, estuarine and marine environments. In order to preserve the economic, cultural, recreational, and ecological value of these environments, it is important that the effects of land use change on water quality are well understood. This thesis examines the nitrogen yield into the Tauranga Harbour, on the east coast of the North Island of New Zealand. The Soil and Water Assessment Tool (SWAT) model was used to examine the nitrogen yield from the Wairoa River under the present land use, as well as two hypothetical scenarios consisting of either only indigenous forest or agriculture. The Wairoa River represents more than 50 percent of discharge into the harbour. In addition to the numerical modelling, recorded data from a number of catchments in the area were also analysed to examine the spatial distribution of nitrogen yield into the Tauranga harbour. Analysis of both the simulation output and recorded data shows an increase in nitrogen yield with increased agricultural area. The recorded data also shows a strong relationship between urban and industrial area and nitrogen concentration. Long term trends are difficult to determine using the recorded data due to the high variability in nitrogen yield that is shown in the SWAT model output. Model outputs from different land use scenarios show changes in the temporal cycle of nitrogen yield. Increased agricultural area caused nitrogen yields to increase during winter, and decrease during summer. Increasing the indigenous forest area had the opposite effect, reducing nitrogen yield over winter, and increasing yield over summer.