With the growth of urban areas and climate change, decisions need to be taken to improve water management. This paper reports an assessment of the impact of greywater recycling systems on catchment scale hydrological flows. A simulation model developed in InfoWorks CS (Wallingford Software Ltd) was used to evaluate how river flows, sewer flows, surface runoff and flooding events may be influenced when grey water recycling systems of different number and scale are implemented in a representative catchment. The simulations show the effectiveness of greywater recycling systems in reducing total wastewater volume and flood volume. However, no hydraulic impacts due to implementation of greywater was identified by the model.