The oxidation of the iron sulfide, pyrite, in acid sulfate soil flood plains generate substantial acidity and this acid has caused further weathering of the soil profile. The movement of groundwater from these soils is an important geochemical control on surface water quality. The flux of acidified and metal-rich water during a wet season rainfall event has been examined at two study catchments on the Tweed River in eastern Australia. At the sites, 81 kg/ha and 60 kg/ha of oxidisable acidity are exported, along with Al, Fe and Zn during the flood event. The main contributors to the acid flux are H+, Fe and Al at the first site and whilst Fe and Al are present in the drainage waters at the second site, the main contributor is likely to be H+. The different flux characteristics at the sites may be caused by different surface soil hydraulic conductivities and oxidation history.