The Wairau Freshwater Management Unit (FMU) is defined by the catchment of Marlborough’s largest river, the Wairau River. The river’s catchment is some 4,187 km2 and encompasses about one-third of the district’s land area. The FMU extends the full length of the region from the western mountainous region through to the east, reaching the coast at Cloudy Bay. To the north are the Te Hoiere/Pelorus and Marlborough Sounds Complex FMUs. To the south are the Waiau-toa/Clarence and East Coast Complex FMUs. The Wairau River has many tributaries. Those to the north flow relatively short distances from the Richmond Ranges to join the river on the Wairau Plain, while the tributaries to the south are generally longer flowing from the Raglan Ranges.
The upper catchment is dominated by native bush, while the middle reaches have plantation forestry on the lower slopes with pastoral farming in the valleys. Moving further down the catchment, the valley widens into the Wairau Plain where agricultural land use is dominated by viticulture. The Wairau Plain has extensive groundwater aquifers that are essential for providing water to the agricultural sectors, as well as for drinking water and commercial/industrial uses. To the southeast of the FMU is the extensive Wairau Lagoons, which are both ecologically and archaeologically significant in the region. The Wither Hills also make a prominent southerly backdrop to Blenheim and are widely used for recreation.