A methodology for the estimation of household potable water saving due to internally plumbed rainwater tanks (IPT) is presented in this paper. The methodology is based on a pairwise comparison of household water billing data between homes with IPT and without rainwater tanks (No Tank). These savings were compared with estimations using measured end use data and rainwater demand predictions using the Rainwater TANK model. The paper describes the application of this methodology to a case study in the south-east Queensland (SEQ) region of Australia. There was a significant reduction in mains water consumption for IPT properties in all regions studied in SEQ. Water reductions from mains supplies varied markedly across regions with mean values ranging from 20 to 95 kL/hh/y with an average mean of 50 kL/hh/y. Median water consumption values, ranged in mains water reductions from 28 to 52 kL/hh/y, with an average median of 40 kL/hh/y. Considering both measures an average water saving between 40 and 50 kL/hh/y can be expected from internally plumbed rainwater tanks. Water restrictions appear to have a strong influence on estimated reductions in mains water use. In regions where water restrictions were severe, water consumption was less varied between No Tank and IPT homes with a consequent reduction in estimated savings observed. Recommendations for further work include a survey to capture confounding factors that could not be fully controlled in the desktop study and a controlled pairwise experiment to monitor water consumption from raintanks.