Scyphozoans of the genus Cassiopea are notable for their unusual benthic habit of lying upside-down with their exumbrella resting on the substrate and oral arms facing upwards resulting in their common name “upside-down jellyfish”. Cassiopea includes species that have been historically confused because of taxonomic ambiguity. Additionally, some species are considered to be invasive, which can have significant economic and environmental consequences by impacting fisheries, tourism, and trophic structures. In temperate southeastern Australia, Cassiopea medusae were first reported in temperate Wallis Lake and Lake Illawarra in 2016, and then Lake Macquarie in 2017, though historically these jellyfish have a more northern tropical distribution in Queensland, eastern Australia. Owing to the invasive potential of Cassiopea, correct species identification is crucial for future management. To address this knowledge gap, this study used genetic comparison through the cytochrome c oxidase subunit I (COI) barcoding gene and morphometric analysis, together with revision of type and topotype material of Cassiopea maremetens Gershwin, Zeidler & Davie, 2010, an incompletely known nominal species from Queensland, to investigate the identity of Cassiopea occurring in Lake Macquarie. The morphometric analysis was also used to identify key features that distinguish the Lake Macquarie species from a second species, designated Cassiopea sp.3, that is also expanding its range southwards in eastern Australia, and which may be sympatric in some areas. The results of this study show the species occurring in Lake Macquarie is Cassiopea xamachana Bigelow, 1892, originally described from Jamaica and subsequently widely reported from the Western Atlantic and the Indo-West Pacific. Additionally, we demonstrate that Cassiopea maremetens, is a junior synonym of C. xamachana. Morphological characters that can be most readily used to distinguish mature specimens of C. xamachana from C. sp.3, which has an overlapping distribution on the Australian east coast, are: (1) the number of large appendages on the oral disc, which is much higher in Cassiopea sp.3 (at least 1 but up to 14) vs. a maximum of two in C. xamachana; (2) the oral arm branching pattern, which is usually alternating for C. xamachana, but a combination of alternating, bifurcating and pinnate for Cassiopea sp.3; (3) the length of the large appendage on the oral arm, which is proportionally longer relative to the bell diameter in C. xamachana.