This paper reports a study aimed at quantifying the yield from rainwater tanks in South East Queensland (SEQ) by considering the spatial variability of tanks. The methodology involves Monte Carlo simulation of tank storage behaviour by considering the spatial variability of input variables, ie, tank sizes, inflows to the tank, household water demand and losses associated with the tank inflow. Probability distributions and probability based methods have been used to represent the spatial variability of input variables. The results have indicated that the error introduced to the average annual yield of a system with multiple internally plumbed rainwater tanks can be in the order of 14% when spatial variability is ignored. For single family new residential households (ie, detached dwellings) in SEQ, the rainwater tank yield found through stochastic simulation is 37 kL/h/y, if the tank is used for the same household end uses. Further work is in progress to include a large sample of tank sizes and roof areas as well as water uses of both new and existing single family residential houses in Gold Coast, Ipswich and Sunshine Coast.