Abstract The factors determining the concentration and speciation of copper in the waters and sediments of Macquarie Harbour, Tasmania were investigated. This harbour is the most extensively copper-contaminated estuarine water body in Australia owing to current and historical inputs of metal-rich waters and sediments from the nearby Mount Lyell copper mine. The dissolved copper concentrations in the harbour water column were highly variable (4-560 姠l-1) and displayed a north to south gradient, decreasing with distance from the King River, which carries the inputs from the mine. The most significant process affecting dissolved copper concentrations was the neutralisation of acidic river waters with seawater and the resulting coprecipitation with iron oxyhydroxide flocs. Approximately 60% of the riverine dissolved copper input was removed from solution by this process. Particulate copper concentrations in surficial benthic sediments were high in most regions of the harbour (typically 0.5-1 mg g-1). In the north, sediments were dominated by fine, mine-derived material and showed uniform particulate copper concentrations with depth. Sediment acid-volatile sulphide concentrations were highest (11-142 孯l g-1) in the southern harbour and were barely detectable in the northern harbour region (<0.46 孯l g-1). A similar north-south gradient of sediment organic carbon concentrations was observed. Very high porewater concentrations of copper (up to 520 姠l-1) and iron (200 mg l-1) were found at sites in the northern harbour. The high porewater copper concentrations are believed to result from the oxidation of porewater Fe(II), formation of amorphous iron oxyhydroxide and the associated pH-related dissolution of particulate copper. Calculations indicated a positive flux of dissolved copper from the sediments at sites in the northern harbour. However, in the southern harbour, the high acid volatile sulphide concentrations of the sediments meant that they acted as a sink for dissolved copper, resulting in low porewater copper concentrations (<1-10 姠l-1) and a significant copper flux from the overlying water to the sediment. The study illustrates the roles of iron redox chemistry, associated pH gradients, and acid volatile sulphide in controlling copper mobility in contaminated estuarine environments.