Abstract: Waste stabilisation ponds (WSPs) use natural microbiological, photosynthetic, biochemical, physico-chemical and hydrodynamic processes to treat wastewater. They require little technical attention during operation and are less demanding in terms of construction cost and energy consumption than other engineered wastewater treatment systems. Practical engineering experience and research over the past few decades have established that hydrodynamics is crucial in determining the treatment efficiency of WSPs. After reviewing a large number of pond systems operated in Australia, Wood et al. (1995) stated that many systems were found to operate below an optimal level due to a reduced retention time of wastewater. It is therefore required that the hydrodynamic retention time distribution be precisely understood so that the hydrodynamic behaviour and the overall treatment efficiency of WSPs can be accurately evaluated.