GIS-based analyses of relevant time-series data (for example, analogue and digital hydrographic data and aerial orthomosaic imagery) for both the Gippsland Lakes and Snowy River coastal lagoon system entrances (located in East Gippsland, Australia) shows that in both cases, coastal zone morphology changes have been significant. Within the respective contributing catchments, influent streamflow discharges have progressively fallen, due to the combined effects of large-scale appropriation of fresh water for agricultural, urban and industrial use, and a reduction in catchment rainfall. The observed and quantified time-series coastal morphology changes reported in this paper are primarily a consequence of the diminution of ebb-tide volumes at the respective ocean entrance areas, due to the documented reduction in catchment streamflow discharge. In both cases, relevant coastal management authorities have progressively had lagoon entrance maintenance problems imposed upon them by upstream catchment decision-makers. Future combined demographic and climate change scenarios for south-eastern Australia are likely to exacerbate this issue. It is argued that a more effective regional integration regime between catchment and coastal zone management is called for, not only at policy level, but with regional/local decision-support based on detailed geography maintained in GIS.