Pesticide runoff from agriculture poses a threat to water quality in the world heritage listed Great Barrier Reef (GBR) and sensitive monitoring tools are needed to detect these pollutants. This study investigated the utility of passive samplers in this role through deployment during a wet and dry season at river mouths, two near-shore regions and an offshore region. The nearshore marine environment was shown to be contaminated with pesticides in both the dry and wet seasons (average water concentrations of 1.3-3.8 ng L(-1) and 2.2-6.4 ng L(-1), respectively), while no pesticides were detected further offshore. Continuous monitoring of two rivers over 13 months showed waters flowing to the GBR were contaminated with herbicides (diuron, atrazine, hexazinone) year round, with highest average concentrations present during summer (350 ng L(-1)). The use of passive samplers has enabled identification of insecticides in GBR waters which have not been reported in the literature previously.