A combination of spatial modelling, sediment tracing techniques and water-quality data were used to determine the major source of fine sediments in the Herbert River catchment, Queensland, Australia. Using modelling, hillslope erosion was predicted to be the dominant source of sediment, contributing 52% of the total sediment load at the estuary. Gully and stream bank erosion contributed equal loads to the estuary (~24%). The 137Cs concentrations measured in this study support the modelled predictions for contributions from different land uses. Results from modelling and sediment tracing also predicted similar ratios of hillslope to channel erosion. The total suspended sediment loads predicted for the downstream freshwater limit of the catchment are within 10% of longer term measured values. These results suggest that the modelling approach used in this study is useful for determining sediment budgets for large tropical catchments.