Between 2016 and 2024 Sydney Water conducted the Wet Weather Overflow Monitoring program to understand the impact of Wet Weather Overflows (WWOs) on the environment, with new learnings building upon a previous risk assessment of Sydney Water discharges from the late 1990s. Toxicity testing, together with tracking contaminants of concern from both influent and the receiving water samples, formed lines of evidence to assist in the interpretation of morphometric data obtained from freshwater macroinvertebrate samples. Insufficient dilution of WWO spills from emergency relief structures (ERSs) was attributed to adverse ecological effect observed in morphometric data under three receiving water situations. These were represented by: too many ERSs spilling to same point of an urban stream; an oversized ERS spilling to a very small urban stream; or too many spatially separated ERSs spilling to a small urban stream reach. Studies tracking contaminants identified ammonia as a contaminant of potential concern. Companion direct toxicity assessment and toxicity identification and evaluation testing of samples, collected under differing weather conditions, established ammonia to be the primary cause of toxicity. Ammonia concentration results evaluated against the ANZG (2018) guideline value (95% species protection) best represented toxicity testing outcomes. These WWOM investigations found that the risk of adverse ecological effect is reduced under increased dilution from inflow and infiltration of rainwater ingress into the sewer system. Follow up toxicity testing established that receiving waters need to be of an adequate capacity to achieve a greater than 2x dilution of a WWO spill to remove the risk of an adverse ecological effect. A stepped approach was recommended to identify potential low dilution stream settings, and deployment of sondes to collect ammonia data, would establish where WWO spill volumes pose a risk of adverse ecological effects to inform prioritisation for capital solution planning to abate WWOs.