Treatment wetlands can be a viable wastewater treatment option, especially in rural and remote regions where centralized wastewater treatment is not feasible. Bacteria fate and transport modeling within wetlands requires further development if they are to become a more reliable and predictable form of wastewater treatment. The goal of this paper was to calibrate and test an unsteady state numerical model for the simulation of E. coli fate and transport within full-scale surface flow (SF) wetlands treating domestic wastewater. The Water Quality Analysis and Simulation Program (WASP) was used to develop the model. Accurately predicting E. coli effluent concentrations using WASP was difficult due to the dynamic nature of the wetland environment including hydraulics, seasonal variability, and wetland maturity. WASP was successful in predicting average E. coli effluent concentrations but did not accurately forecast maximum and minimum values. The model produced better fits with observed E. coli effluent concentrations during the summer months, when observed effluent concentrations were less variable. Hydraulic tracer studies and model results suggest that preferential flow pathways may be affecting E. coli removal due to reduced retention times. Flow channelling or short circuiting may have been caused by high flow conditions and/or dense cattail growth. A more detailed understanding of treatment wetland hydraulics is required before we can accurately predict treatment performance.