Established urban catchments often result in increased flow and reduced coarse-grained sediments to receiving streams. The resulting loss of coarse-grained sediments (e.g. sands, gravels etc.) in streams has negative consequences for aquatic ecosystems, including reduced foraging and refuge for macroinvertebrates and fish. Stormwater harvesting treatment systems often exacerbate the problem. Gross pollutant traps (GPTs) are commonly designed to remove particles and particulate associated contaminants from reaching to water body, further starving streams of coarse-grained sediments. A concern for passing coarse-grained sediments through these systems, however, is the associated pollutants that would also be transported to the receiving water body. The aim of this study is to determine particle size distribution, pollutant concentration and the effect of particle size on pollutant concentration before and after gross pollutant traps. We review the literature, and present a case study on a GPT at Wicks reserve in Victoria. We compare the toxic potential of sediment associated contaminants with quality guidelines to understand the potential toxicity on aquatic organisms. Based on the result, the treatment systems may be more effectively designed for specific sediments sizes, with the high pollutant concentrations captured and low pollutant particles redirected back to the stream. This will help us consider alternative designs to mitigate pollution, maintain stream health and reduce the cost of maintenance.