A two-year study was conducted at the Narellan Creek Catchment between September 1998 and August 2000. To improve the quality of the water that flows in to the Nepean River, the Harrington Park retention basin was enlarged and rehabilitated from 7.0 ha to 12.0 ha area. This study measured several parameters of water quality to assess the efficiency of the Harrington Park retention basin for water quality improvement. Three sampling sites – the Camden Valley Way (CVW), Harrington Park retention basin (HPK) and Kirkham Lane (KLN) were selected for sample collection. Water flows from CVW to HPK then to KLN and finally to the Nepean River. Rainfall, water level, discharge rate and ambient temperature were recorded at the KLN site while the selected water quality parameters were determined at all the three sites. Low flow samples were manually collected at monthly intervals. An auto-sampler which collected water samples during rain events (for high flow data) was set up at the KLN site. Rising stage samplers which collected water at different water levels during a rain event, was set up at all the sites but data from this device was found to be of limited use due to the varying time lags between the filling of each bottle. Statistical analyses by various measures show that the Harrington Park retention basin played a significant role in improving the upstream and in-stream water quality. These study shows that constructed wetlands like the Harrington Park retention basin has the ability to significantly improve the water quality of a catchment. The water quality improvement is greater at lower water flow rates. / Doctor of Philosophy (PhD)