Pesticides enter the Great Barrier Reef (GBR) from urban and rural run off. We quantify diuron in the GBR using the eReefs marine model (1 km grid resolution) coupled with the GBR-Dynamic SedNet catchment model from 2016 to 2019. Diuron is dispersed from the coast via freshwater river flow and is advected and diffused by the hydrodynamic model which includes 3-dimensional distributions of velocity, temperature, salinity, sea-level, wind, atmospheric pressure gradients, water fluxes, tides, currents and waves. We first compare diuron from the eReefs marine model with diuron observations throughout the GBR validating our approach. We then quantify diuron dispersal from catchments into the coastal areas of the GBR, highlighting areas above the present Default Guideline Value (DGV: 90% species protection, 430 ng L-1) and compare with the Proposed Guideline Value (PGV: 99% species protection, 75 ng L-1). While diuron dispersal is highly localised to the river plumes and inshore environments, diuron concentrations below the DGV extended northward along the coast. The eReefs marine model is well suited to visualise concentrations, including below environmental threshold values, over a large regional area at a fine temporal scale and can highlight potential areas at risk and where monitoring would be most effective.