On the morning of the 21st of August 2017 Tweed Shire Council officers awoke to find a large tidal anomaly on the Tweed River had passed through. This resulted in saline intrusion over the weir at Bray Park and into the local drinking water supply. Although this was a spring cycle in the tidal phase for the region there was no significant forecasted wave action accompanying the high tides and no foreseeable cause for Council to enable its weir topping prevention protocol. In Australia such tidal anomalies can be caused by coastally trapped waves (CTWs) which are propagating waves caused by low pressure systems that originate in the south-west of Australia and travel east. Using the NSW network of tide and river gauges this CTW event was tracked up the coast of NSW and into the Tweed area where effects of the weir inundation were felt most. A resulting outcome of this event has seen Tweed Shire Council utilise this network data and other predictive models to better improve weir overtopping procedures.