Field testing of a proprietary stormwater treatment device (GPT) was undertaken over a one year period at a commercial site located in Sippy Downs, Queensland. The focus of the study was primarily on evaluating the effectiveness of the GPT device in removing pollution in the form of nutrients (Total Suspended Solids, Total Nitrogen, Total Phosphorus) from stormwater runoff. Water quality analysis was performed on water samples taken from the inflow and outflow of the GPT during 15 natural rainfall events. A new testing protocol was developed to ensure a comprehensive investigation of the stormwater treatment performance of the GPT. Pollution treatment Efficiency Ratios (ER) calculated for the GPT were found to be 49.2% for TSS, 26.6% for TN and 40.6% for TP. Although the nutrient removal rates of the GPT observed in the study were below those specified by Queensland regulations, the results are considered notable for a stormwater treatment device that was not specifically designed to remove nutrients from stormwater.