Prehistoric depositional signatures for large-scale washover involving marine inundation events such as storm and tsunami have been the subject of considerable research over the last 15 years. Much of this research has focused on the identification of sand sheets in back-barrier environments as depositional records for extreme washover events. All these deposits must have a sediment source, and by their nature, the most likely source of sediment for washover into back-barrier environments is the barrier itself. This study identifies an erosional signature for large-scale washover from a small coastal barrier on the southeast Australian coast. A distinct lense of marine sand, up to 90 cm thick, is confined vertically by peat and is found in the upper fill of a closed freshwater back-barrier lagoon sequence. This sand lense is attributed to a large-scale washover event that occurred at approximately 1500 AD, possibly by tsunami. The hypothesis for this study was that any event that breached the dune system must have caused considerable geomorphic change to the dunes and hence may leave an erosional signature of the event. Ground penetrating radar transects of the system show an erosional contact between a series of truncated pre-event dunes and several small overlying post-event dunes. This study outlines a relatively simple non-invasive method for the identification of an erosional signature for prehistoric largescale washover by storm surge, exceptionally large waves or Tsunami.