Analysis of historical archives has uncovered reports of extreme storm surge inundation and overland flow associated with the 1921 tropical cyclone impacting Shark Bay on the mid north coast of Western Australia. Anecdotal inundation heights of ~7-10ft (2-3m) in the town of Denham on the western bank of Denham Sound, 20ft (6m) on the eastern bank of Denham Sound, and 10ft (3m) at the southern end of Freycinet Estuary, coincided with a spring tide (~0.9m AHD). The overland flow associated with the storm tide extended more than 6 miles (9.66km) inland at the southern end of Freycinet Estuary, as evidenced by saltwater inundation of freshwater wells and accounts of sharks and fish stranded inland. A comparison of these historical inundation heights is made with those in a 10000-year synthetic tropical cyclone storm tide climatology of southern Western Australia), that generated using a Monte Carlo tropical cyclone track model, a parametric cyclonic wind model and a high resolution hydrodynamic model in Delft-FM (Burston et al., 2017). The 1921 tropical cyclone generated storm surge and storm tide exceeding the current 500-year ARI planning level for Denham, indicating either that the storm surge risk at long return intervals far exceeds the risk at planning time frames or that the planning risk level is underestimated for this site. We hypothesise that the amplified storm surge associated with the 1921 event is due to the storm surge wave travelling at the same celerity as the tropical cyclone itself. This hypothesis is tested using a set of sensitivity model simulations of the event on the Delft-FM hydrodynamic model. The triggering of such a process is difficult to assign an ARI value, and emphasises the importance of understanding the Probable Maximum Event for risk management and planning.