A two-year study of the relationships between water chemistry, hydrology and climatology in areas of acid sulfate soils has demonstrated that discharge of sulfuric acid to estuaries is not dependent solely on the magnitude of a rainfall event. Large rain events did not always produce significant changes to water chemistry, and small rain events could produce large changes if the prevailing conditions were suitable. The magnitude of changes to estuarine waters was found to be dependent on the position of the watertable, and therefore the available soil pore space, and the store of acidic water in floodgated drains at the time of rainfall. These results have significance for predicting the discharge of acid drainage to estuarine environments and the possible impacts on aquatic organisms. A water balance equation, which estimated acid discharge to surface waters as high as 317 t of H2SO4 in one month, can be used to predict monthly discharges for rain events of varying magnitude if prevailing weather conditions are known.