Water security has become a major issue within Australia and particularly within South East Queensland (SEQ). In response to projected limits to conventional water supply systems in SEQ a range of innovative projects have been initiated. A key element of this water security response has been the development of the SEQ Water Grid, which includes the capacity for introducing Purified Recycled Water (PRW) into the region's largest water supply reservoir, Lake Wivenhoe. The highly dendritic and thermally stratified nature of Lake Wivenhoe suggests that three-dimensional (3-D) flows are likely to exert a controlling influence on PRW mixing. In this investigation we present the results of initial 3-D simulation of PRW mixing in Lake Wivenhoe with a focus on the boundary condition used to simulate PRW inflows.