A simple empirical model is presented which captures the relationship between changes in total water storages within the Murray Darling Basin system and annual rainfall across south-eastern Australia. Despite its simplicity, it is able to capture over 90% of the variance in observed storages over the past 13 years. The utility of such a model is demonstrated by providing a simulation of storages for both the past 100 years based on observed rainfall, and for the next 100 years based on the ouputs from climate model simulations. Importantly, it provides a basis for incorporating the effect of relatively slow, but persistent, changes in rainfall on top of the large interannual and interdecadal variations that are a feature of Australian climate. A feature of the model is that, while changes in total storages are linearly related to rainfall, the existence of thresholds causes the average ratio of percentage changes in storages to percentage changes in rainfall to be close to 3 - a value consistent with estimates of the relationship between rainfall and runoff across Australian catchment regions. Estimates are presented of possible changes to mean storage levels, frequency of overflows and frequency of underflows that could occur between the 20th and 21st centuries. The results tend to highlight the importance of changes in mean annual rainfall at the large scale compared to changes at shorter and smaller spatial scales when estimating plausible impacts. While a 10% reduction in rainfall has serious consequences, it is possible that changed management practices could ameliorate the effects of this over the short to medium term. A 20% reduction appears to much more serious and the consequences far less manageable.