This paper examines data on actual reductions in consumption of water supply due to the widespread installation of rainwater tanks at residential properties in the Sydney metropolitan area and surrounding areas connected to Sydney Water Corporation water supply mains. The water consumption was based on metered potable water usage between 2002 and 2009. The number of properties in the study database totalled 962,697 single residential dwellings. Of this a total of 52,576 or 5.5% of Sydney’s households had a rainwater tank registered with Sydney Water Corporation.  The water usage consumption before and after the installation of the rainwater tank was analysed to quantify the extent to which rainwater tanks reduced mains water consumption. The average percentage of water savings by installing rainwater tanks across all 44 local government authorities was 9%. In some Sydney localities this reduction was up to 15%. On average, a household was able to save around 24 kilolitre of water annually by installing a rainwater tank even without considering other factors that affect water usage. The results were compared against socio-demographic factors using variables such as household size, educational qualifications, taxable income, rented properties, and non-English-speaking background, etc. to gain an appreciation of how these factors may have influenced the outcomes evident in the data. Among the co-relations found were that most properties within inner Sydney with a rainwater tank achieved at least a 9 to 11% additional reduction in water usage, with more than half of those local government authorities achieving more than 11%; properties with larger land area were more likely to have a rainwater tank installed; local government authorities with more people born in non-English speaking countries had lower reduction in water consumption reductions.