The Sydney Harbour catchment area includes a number of different river systems which drain into the Port Jackson estuary. During high rainfall events the catchment areas behave differently to deliver contaminants into the Port Jackson estuary. Under conditions of high rainfall contaminants are delivered to areas very quickly as freshwater slides over the top of the denser more saline waters of the estuary. Secchi Disc samples were taken along the Port Jackson estuary over four days to determine the behaviour of the freshwater plume as it travels in a layer of freshwater to be flushed or flocculate out at some point in the estuary.