Between August 2014 and August 2016, a long-term monitoring study of saline mixing within a shallow coastal aquifer along the west coast of New Zealand’s lower North Island was carried out. Two main methods were used in order to determine the seawater intrusion potential and dominating seawater mixing processes at the site of interest. With geoelectrical time-lapse measurements along nine 2D profiles it could be shown that there is significant change in resistivity (± 40%), which correlates well to a seasonal change in total dissolved solids. Additionally, it was possible to show clearly opposite behavioural patterns in seasonal aquifer response between profiles situated in rural areas compared to those located in urban areas. Study of the groundwater chemistry with in-situ multi-probe and laboratory based ICP-MS measurements in 18 domestic bores was able to determine saltwater intrusion as a function of bore depth and distance from the coast. This confirmed the hydrogeological stratigraphy inferred from the geoelectrical models and supported the hypothesis of evapotranspiration/crystallisation being the dominant process within the saltwater affected part of the aquifer during summer as was drawn from the geophysical study. The combination of these methods clearly leads to a broader understanding of an aquifer system and is superior to using each method exclusively. Furthermore, the results of this monitoring project add significantly to the current knowledge about the dynamic behaviour of saltwater-freshwater systems in coastal aquifers which can be used for groundwater assessment in similar hydrogeological areas around the world.