Stable isotopes (deuterium, carbon-13 and oxygen-18) and radiocarbon (carbon-14) have been used in conjunction with chemical data to evaluate recharge mechanisms, groundwater residence time and palaeohydrology within the confined Dilwyn Formation sand aquifer in the Gambier Embayment of the Otway Basin. This aquifer does not receive recharge down-gradient of the hydraulic hinge-line, so data has been interpreted along two discrete flow lines. The mean residence time of groundwater (determined by carbon-14) in the confined aquifer from the hydraulic hinge position to the sea (a distance of about 50 km) along an inferred flow path is approximately 12,800 years. The corresponding hydraulic travel time calculated from Darcy's law is approximately 49,000 years. The apparent discrepancy may be a result of eustatic sea level lowering during the last glacial. Because the groundwater system is hydraulically connected to the sea, lower sea level would result in increased gradients and a decrease in groundwater residence time. Variations in stable isotopic composition along flow lines suggest a number of recharge mechanisms. Stable isotope data indicate progressive depletion of deuterium and oxygen-18 in the groundwaters over the past 30,000 years. Groundwaters older than about 10,000 a BP were recharged either during a cooler climate climatic regime (lower precipitation / evaporation and temperature) and/or the vapour source(s) had travelled over a greater continental mass than those recharged over the past 10,000 years. Lower chloride concentrations in some of the older groundwaters indicate lower evapo-transpiration rates in the recharge areas prior to 10,000 years ago.