The Great Barrier Reef (GBR) Marine Monitoring Program (MMP) was established in 2005 to monitor the inshore health of the GBR. A range of pesticides, including herbicides and insecticides, have been monitored as part of this program using passive sampling techniques. Currently, the MMP provides information of these pesticides on the extent of contamination, spatial and temporal variability, and potential threats to the biota in the GBR lagoon. However, long temporal trends during the entire monitoring (2005 - 2018) have not been assessed. The assessment of temporal trends is relevant as it can provide insights into the effectiveness of regulatory actions and management over time. We used 13 years of monitoring data of five PSII herbicides (ametryn, atrazine, diuron, tebuthiuron, and hexazinone) to conduct temporal trend analyses at 11 monitoring inshore sites within the GBR lagoon. Results from the trend analyses of over 43 time series suggest increasing tendencies for the five PSII herbicides at all the 11 monitoring sites, with 18 time series showing significant increasing trends. Six time series out of the 16 were shown to be robust enough to validate moderate annual changes of 11% to 20%, meaning that the PSII concentration doubles in about four to six years if emissions continue at the same rate. Estimation of smaller trends, like 5% annual changes (with a statistical power of 80%) of the five PSII herbicides at the monitored sites will require a median of 22 years of monitoring.