Parallel research programs encompassing the hydrodynamics and geochemistry have been undertaken in Oatley Bay, a small urban estuary that is a lower tributary to the Georges River in Southern Sydney Using field data and a simple numerical model, the regimes of flow effecting circulation in Oatley Bay have been determined. The efficiency of flushing and the subsequent fate of conservative pollutants are discussed. The field data comprised bathymetry, tides, drogue tracking and dye tests, the latter enabling the calibration of the model under different environmental conditions. With this model we have performed numerous runs and have characterised temporal and spatial patterns in the estuarine circulation. These results have been collated in terms of a simple dimensionless parameter – the estuary flushing parameter. This will allow generalization of the results for application to other similar estuaries.