Fish kills have been reported from both the Tweed and Macleay Rivers on the NSW north coast. Pesticides from canefields, sewage overflow and mine runoff had all been suspected, but without foundation. Researchers found that at times of flooding, iron pyrite was released from the soil, and then oxidised to form sulphuric acid and so create an actual acid sulphate soil. The acid affects aquatic food chains, reducing populations of fishes and crustaceans. Methods to control the acid sulphate soil problem are outlined.