The hydraulic and morphological response of a large coastal lake system on the Australian east coast is assessed in regards to rising mean sea levels. The relative influence of sea level rise in relation to other significant anthropogenic perturbation to the ocean inlet is investigated. Numerical modelling and other techniques are used to predict the likely behaviour of tides, storm surge and estuarine flushing using a predicted morphology of the lake following sea level rise. The analysis shows that the tidal range within the lake is expected to increase and is associated with a significantly larger tidal prism. The predicted increase in the volume of the tidal prism is expected to be primarily attributed to the continuation of inlet scour currently occurring in response to entrance training, however, sea level rise is anticipated to reinforce this behaviour. Higher relative extreme water levels are predicted in the lake during storm surge events due to the increased hydraulic efficiency of the predicted inlet channel. Lake flushing is likely to be enhanced, possibly improving the water quality of the estuary but altering estuarine ecosystem characteristics. The finding of this study have significant ramifications for the planning and management of this and other estuaries and associated floodplains in relation to both the built and natural environments. Specifically examples include; the extent of land subject to inundation risk, future navigational dredging requirements and the impact on fringing ecologies.