Wallaga Lake is an estuarine lake in Bega Valley Shire in New South Wales, Australia, the largest lake in southern NSW. It is located between Bermagui to the south and between Tilba Tilba to the north, situated beneath Mount Gulaga, in the traditional lands of the Yuin people. A large section of its foreshore and catchment are within the Gulaga National Park, since Wallaga Lake National Park, Goura Nature Reserve, and Mt Dromedary Flora Reserve were combined into the larger national park. There is an island in the lake known as Merriman's Island, Merriman Island or Umbarra.