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Rainwater is regarded as an alternative water source for water supply augmentation in metropolitan areas to mitigate rising water stress caused by increasing population and climate change. There exists a knowledge gap in the evaluation of rainwater-harvesting systems to increase urban water supply reliability. This study provides insight into the impacts of developing rainwater-harvesting systems to improve urban water supply reliability by modeling joint water supplies from both decentralized rainwater-harvesting systems and centralized stream reservoirs. The model is then applied to Melbourne, Australia, using two rainfall regimes. The results show that enlarging roof areas and rainwater tanks, especially for new homes, can increase volumetric rainwater supply reliability at the household scale. This not only increases supply for households but also increases water supply reliability of stream reservoirs at the regional scale. An average roof area of 200 m2 and an average tank size of 8–10 KL can harvest enough rainwater in Melbourne to increase reservoir water supply reliability to 90% for current urban water demand during an extended drought. Using these results, efficient sizes for rainwater tanks are determined by analyzing the cost of enlarging a tank compared with its marginal benefit of increasing rainwater supply reliability. Furthermore, the impacts of climate variability and demand seasonality on system performance are explored using storage-reliability-yield curves for rainwater-harvesting systems. These results provide general insights regarding the feasibility of rainwater water supply augmentation constrained by tank size, roof area, rainfall regime, and demand seasonality. The proposed method for coupling a rainwater tank model at the household scale with a reservoir model at the regional scale is transferable to the design of regional rainwater-harvesting strategies in other cities. [ABSTRACT FROM AUTHOR]