The Macleay Catchment in southeastern Australia is home to one of the largest known antimony (Sb) environmental dispersion plumes in the world. Historic mining practices in the fluvially energetic upper catchment has resulted in several thousand tonnes of Sb and As entering the river system. This plume was discovered during baseline geochemical sampling of the catchment 20 years ago, initiating a long term, concerted research effort to understand all aspects of the environmental fate and behaviour of Sb in the catchment. This included comparisons with arsenic (As) throughout the investigations. Sediment Sb concentrations ranged from several thousand mg kg^-1 in the high energy upper reaches of the catchment, with values of tens of mg kg^-1 300 km downstream on the coastal floodplain. Arsenic distribution and relation to flood events partially diverged from this pattern in the lower catchment. Both elements showed strong associations with flood patterns in the coastal floodplain via variogram analysis and calculation of sediment deposition locations, but dating of sediments suggested As had a secondary source in the floodplain. Values in river water followed similar distribution patterns with values hundreds to thousands of times greater than relevant human and environmental guideline values in the upper catchment. These values vary strongly with flow regime and season. Uptake into a range of vegetables and pasture species was low for both metalloids, with bioconcentration factors ranging from 0 to 0.14. Bioconcentration factors were consistently higher for As than Sb. Bioaccessibility in selected soils was typically <3% for Sb, and slightly higher for As (3.7-6.1%). This patterning was reflected in food web analyses, in which biodimunition of both metalloids was evident between trophic levels, but to a greater extent for Sb than As. Arsenic association with oxyhydroxide phases appears stronger throughout the catchment, with similar associations playing an increasingly important role for Sb in the coastal floodplain. Overall, from the same primary source and on the same time scale, both metalloids have dispersed hundreds of kilometres in approximately 150 years, with Sb consistently displaying slower rates of dissolution and re-association with sediments and soil phases, uptake, trophic transfer and bioavailability throughout the catchment, compared with As.