The growing population in New South Wales underscores the importance of finding alternative water sources to supplement the current water supply. The aim of this research is to evaluate the performance of rainwater harvesting systems (RWHS) as an alternative water source in selected NSW suburbs and to suggest suitable rainwater tank sizes to avoid overflowing. The average annual rainfall (AAR) data, collected from Bureau of Meteorology (BOM), in selected suburbs including Hurstville, Blacktown and Bankstown, shows the rainfall patterns are relatively higher during January to June and slightly decline afterwards. Tankulator web-based application has been used for data analysis and simulation exercise, which is based on the historical rainfall data of NSW rainfall gauging stations. Tank capacities of 2, 5, 8, and 9 kilolitres (kL) were chosen to perform simulations using the Tankulator tool. Selection of a 2 kL tank resulted in rainwater overflow of 36% in Hurstville, 45% in Blacktown and 44% in Bankstown. The tank size was subsequently increased to 9 kL, which successfully eliminated overflow in both Hurstville and Blacktown. With this tank size, Hurstville achieved the highest water-saving efficiency at 55%, saving 102 kL of rainwater annually, while the water-saving efficiency was estimated as 50% in Blacktown and 48% in Bankstown with a 9 kL tank size.