Scyphozoans of the genus Cassiopea (Cassiopeidae) are notable for their unusual benthic habit of lying ventral-side up with their oral (feeding) arms facing upwards, resulting in the name, “upside-down jellyfish”. Cassiopea are considered to be an invasive genus, which can have important consequences as jellyfish blooms are known to impact fisheries, tourism and trophic structures. Historically, these jellyfish have had a northern tropical distribution in eastern Australia, but in 2017, were first reported in temperate Lake Macquarie. In this study, we investigate the population dynamics of Cassiopea through 3 years of fieldwork, observing their seasonality and distribution patterns. This presentation will focus on their spread within Lake Macquarie and report on their high density during austral autumn, which preliminary results suggest is controlled by a combination of turbidity and temperature. Additionally, this study reports that only females have been recorded within the lake, suggesting there was only one invasion event and Cassiopea have been asexually reproducing. It is important to understand the population dynamics of the invasion in its early stages in order to track its progress and the presented results will aid management strategies to be put in place to control the invasion front if deemed necessary.