The Whangamoa Freshwater Management Unit (FMU) is Nelson's most northerly FMU and the Whangamoa River has the largest catchment. It flows into Tasman Bay at the Kokorua Estuary. The Collins River, Denker stream and Graham stream are the main tributaries feeding into the Whangamoa. The main land use within this catchment is plantation forestry and in the upper reaches there is extensive native forest managed by the Department of Conservation. A small number of coastal catchments are also within the FMU, including Frenchman’s, Omokau and Oananga Streams.

Within the Whangamoa FMU, there are five defined sub-catchments: Upper Whangamoa River (upstream of the Collins River confluence); Lower Whangamoa (downstream of the Collins River confluence); Graham Stream; Collins River; and Denker Creek.

Overall, water quality and ecological health is very good throughout the Whangamoa FMU, indicative of the high degree of native and plantation forestry covering a large proportion of the catchment, though production forest harvesting may be contributing to increased sediment run-off. In some locations, such as the Lower Whangamoa, nutrient levels are elevated contributing to high levels of periphyton growth. Poorer water quality in some lower catchment streams (water clarity, nutrient levels and pH) may be indicative of the effects of pastoral uses and plantation forestry. Key values include natural form and character, recreation for swimming, threatened species habitat, mahinga kai, and trout habitat and spawning.