The Stormwater Management Area-flow overlay aims to protect Auckland's aquatic biodiversity. Auckland has numerous small and narrow streams. Despite their small size, these streams are home to much of our aquatic biodiversity. This biodiversity is threatened by the effects of ongoing urban development. The creation of impervious surfaces in a catchment undergoing development increases the rate and volume of stormwater runoff. This change in hydrology, unless managed, can have a significant adverse effect on streams within the catchment. Increased flows and stormwater volumes can accelerate stream erosion, particularly in steeper upper catchment areas, and can create hydrological conditions that do not support healthy aquatic ecosystems. In developed urban catchments with large areas of impervious surface, increased runoff is one of the primary causes of degraded stream health. However, in areas yet to be developed, or with existing development at low levels, development can be enabled while also protecting and enhancing in-stream biodiversity and other stream values, reducing and managing stormwater runoff, and other measures such as enhancing riparian margins. High-value, and potential value, streams at risk or particularly susceptible to the effects from development have been identified and their contributing catchment areas mapped (stormwater management area: flow (SMAF)). Future development and redevelopment in these catchments will be subject to controls to manage stormwater runoff to enable development, while at the same time protecting Auckland’s aquatic biodiversity from further decline.