A tool to assess the supply risks associated with water allocation strategies employed at effluent reuse facilities is described. The tool is a monthly water balance model and sensitivity analysis. Through analysis of climate records at the Hawkesbury water reuse scheme site (the location of a combined effluent and storm water reuse facility), it was found that an estimate of irrigation demand followed standard statistical distributions. The estimated distribution of irrigation demand was used in conjunction with a water balance model to estimate future storage distributions and hence risks of future over- or under-supply scenarios. The tool is suitable for use in an operational environment to evaluate the influence of demand management strategies.