A WaterNSW risk assessment found that the potential release of the Cyprinid herpesvirus-3 (CyHV-3) may have an adverse effect on water quality in WaterNSW storages. This virus is being investigated by Fisheries as a biological control method for carp. To assess the risk at different hazard levels (carp biomass loadings of 0, 250, 500, 1000, 1200, 2300 and 6000 kg/ha), two small-scale fish kills were conducted in outdoor mesocosms within Prospect Reservoir. In all experimental treatments containing dead carp, there were decreases in DO, increases in nitrogen, phosphorus, pH, conductivity, turbidity, chlorophyll a and phytoplankton biomass which lead to month long algal blooms. Adverse water quality was directly proportional to the biomass of dead carp present in the treatment. In all but the lowest treatment (250 kg/ha), decaying carp reduced DO to zero within a week. Nitrogen (from 0.25 to 1.5-30 mg/L) and phosphorus (from 0.01 to 0.05-5.0 mg/L) concentrations increased dramatically, typically peaking after two weeks. Chlorophyll a levels increased from <5 µg/L to between 100 and 1000 µg/L and algal blooms present in the higher biomass treatments lasted for the length of the experiment (five weeks). Conductivity, pH and turbidity in all treatments with dead carp fluctuated for the duration of the experiment, typically peaking after 10 days of the experiment. The findings of the study have given WaterNSW the opportunity to plan for such poor water quality outcomes, should the release of the virus go ahead as planned.