This dataset was created from the tidal gauging of 51 estuaries in NSW undertaken by Manly Hydraulics Laboratory (MHL) over the period from 1977 to 2005. Of these estuaries, 39 were permanently open rivers and lakes with the other 12 mainly open, but intermittently closed, estuaries also included. Measurement of tidal velocities and water levels was generally over a period of up to 15 hours on a spring tide to capture the maximum flood and ebb tide flows. Spring tides were targeted to capture as much inundation of peripheral tidal flats as possible which is preferable for calibration of numerical models. A number of cross-sections were gauged along each estuary. A mean tidal prism for each estuary has been calculated from the gauging records corrected for the stage of the spring/neap tidal cycle at which gauging was done, the condition of the entrance (shoaled or scoured) and the additional tidal prism, not recorded during the gauging, between the mouth of the estuary and the closest upstream gauging line.; ; Tidal prisms for estuaries were estimated to support the development of a new response-based estuary classification system based on dilution and flushing characteristics. The classification system was used to stratify estuaries in NSW for the development of reference conditions and the design of a sampling program of chlorophyll a, turbidity and associated water quality parameters to assess estuary health.