Scyphozoans of the genus Cassiopea (Cassiopeidae) are notable for their unusual benthic habit of lying upside-down with tentacles facing upwards, resulting in their common name, “upsidedown jellyfish”. In Australia, five named species of Cassiopea have been recorded from the tropical north. Cassiopea are frequently noted worldwide as invasive species and here, we report the first records of the genus and family from temperate eastern Australia on the basis of specimens collected from two widely separated coastal lakes, Wallis Lake and Lake Illawarra; these specimens represent southern range extensions of the genus by approximately 600 km and 900 km, respectively. Cassiopea from Lake Illawarra and Wallis Lake appear to represent different species, which we assign to C. ndrosia and C. cf. maremetens, respectively, noting morphological discrepancies from published accounts.