Reports of pharmaceuticals in STPs and aquatic systems in the northern hemisphere have surged over the last decade. However, the Australian evidence base is relatively limited, and information on the role of seasonal dilution in attenuation of micropollutants is also scarce. We investigated the removal of 11 PPCPs during sewage treatment in Australia's largest inland STP, and concentrations in the effluent-receiving environment under 2 dilution scenarios. Five treatment stages were sampled, as well as upstream and downstream of the effluent outfall in the Lower Molonglo/Upper Murrumbidgee Catchment, which is dominated by effluent flow during dry periods.Compounds of interest include carbamazepine (CBZ), venlafaxine (VEN), sertraline (SER), fluoxetine (FLX), atenolol (ATL), sotalol (SOT), metoprolol (MET) propranolol (PRL), chlorpheniramine (CHP), diphenhydramine (DPH), and triclosan (TCS). Removal of most pharmaceuticals in the STP was incomplete, although the degree of removal was highly variable for compounds in the same therapeutic class, and for the same compounds in different seasons. Removal efficiency was highest for TCS and lowest for VEN (effluent concentrations 5-7 times higher than influent). Influent mass loads and removal efficiencies of cardiovascular medicines varied considerably. Effluent loads were highest for CBZ, VEN and SOT in both seasons (up to 64. g/day). The dilution conditions were clearly reflected in the 'zone of impact' of PPCPs in the catchment. This study confirms that risk assessment models for PPCPs must account for seasonality of influent loads and removal efficiency of STPs, and site validation is critical for predictive capability. Seasonal dilution can play an important role in ameliorating potentially adverse effects related to mixtures of PPCPs in effluent-impacted systems.