The Hawkesbury-Nepean Valley Regional Flood Study is a technical document describing the flood behaviour of the main Hawkesbury-Nepean River from Bents Basin near Wallacia downstream to Brooklyn Bridge, and associated backwater flooding, for existing conditions and under projected climate change. It does not include local catchment flooding or local overland flow inundation. The modelling of infrastructure options to mitigate downstream flooding is beyond the scope of this flood study and is addressed separately. This Regional Flood Study describes the flood behaviour dominated by Hawkesbury-Nepean riverine flooding and its backwater effects. Local catchments are modelled to have the same duration rainfall event as that which causes the highest flood levels in the main river. Shorter duration events are likely to result in higher flood levels within tributaries such as South Creek, requiring separate investigations. Local councils should be consulted to ascertain whether information is available to understand the combined flood risk at particular locations in the valley.