The North Bank area lies on the north side of the Wairau River, Marlborough, New Zealand, bounded by the Richmond Ranges to the north and the Wairau River to the south. The North Bank is an interactive zone where groundwaters and surface waters from North Bank tributary valleys mix with waters of the Wairau River. This investigation aimed to define the nature and origin of groundwaters of the North Bank area. Stable isotopes of oxygen and hydrogen, along with hydrogeochemistry, were utilised in order to define the spatial extent of the North Bank riparian margin and delineate the Wairau River-groundwater interface. Distinct stable isotopic signatures differentiate ground and surface waters that come from high mountain catchments versus those that arrive more locally at lower altitude. The results gathered by this study demonstrated stable isotopes to be the most powerful forensic tool capable of distinguishing Wairau River water from North Bank tributary groundwater sources. In contrast, hydrogeochemical characteristics of the waters of the North Bank were young and chemically dilute in nature, which made them chemically indistinguishable from waters of the Wairau River. Geomorphological mapping was conducted in order to investigate the relationship between groundwater flow patterns and geomorphology upon the North Bank. Geomorphology, in the form of prominent fluvial terraces, was found to play a role in limiting the extent of Wairau River influence to groundwater to either low-lying Q2 Speargrass Formation, Q1 Rapaura Formation alluvium or the Wairau River channel itself. Aquifer pump testing and water level observation carried out in the Waikakaho Valley revealed a plentiful groundwater resource in the local context. Like other tributary valleys within the North Bank study area, surface water and groundwater were found to be chemically and isotopically linked to one another which points to an interconnected ground and surface water resource, larger than first thought. Driven by recharge by the Waikakaho River, the groundwater resource has development potential, and continued monitoring will further define the hydrogeological system and ensure long term sustainable use.