Storm events following a bushfire generate large volumes of fast flowing runoff carrying with it sediments and contaminants (ash, nutrients, organics, and metals) that threaten aquatic environments, recreation and water quality of the River Murray. The main objective of this case study was to assess the cumulative effects of contaminants and to predict risks of multistressors in natural environment due to runoff from bushfire affected areas. During the Australian ‘Black Summer’ bushfires of 2019-20, water samples were collected following storm events from three locations in the Upper River Murray. All water samples were analysed in the laboratory using standard methods and tested for toxicity to water fleas, snails and fish under controlled laboratory conditions. Smothering of freshwater snail egg sacs with ash resulted in complete mortality of freshwater snail (Physa acuta) larvae at TSS concentrations ≥ 300 mg/L. Early life stages of Murray rainbowfish (Melanotaenia fluviatilis) exhibited larval deformities during 12-day exposure. Reproduction was significantly impaired in water fleas (Ceriodaphnia dubia) exposed to bushfire water samples with total suspended solids ranging from 145-530 mg/L. We also explored the applicability of different pulse exposure scenarios to reveal the chronic effects of bushfire runoff to C. dubia. Recovery from multiple pulse exposures was slow and the observed effects were comparable to the continuous exposure scenario. Interactions between metals and high suspended solids played a significant role in modulating the ecotoxic effects of the runoffs. The approaches used in this case study can augment field bioassessment programs and provide information for decision making and management of bushfire affected areas.