Ephemeral saline lakes form along the eastern shore of Spencer Gulf where beach ridges isolate areas of the high supratidal zone. The region is semi‐arid with rainfall averaging about 320 mm/year and with an evaporation rate of 2000 mm/year. Lacustrine sediments in the lake studied contain abundant discoidal gypsum with a calculated δ34S value of +14.9‰ indicative of precipitation from non‐marine, porewater sulphate (δ34S, +13.4‰). Sulphur, deuterium and oxygen isotope distributions and distinctive K/Br and Mg/K ion ratios indicate that groundwaters from landward of and within the lake are non‐marine in origin. The marine‐terrestrial groundwater interface is mobile; after heavy rains, continental‐derived brines flow seaward as far as the high intertidal zone.