The aplysiid sea hare, Syphonota geographica has a predominantly circumtropical distribution. Over the last 15 years, it has spread throughout the eastern Mediterranean Sea where it is regarded as an alien, Lessepsian migrant. Observations from southern Europe and the Middle East illustrate the capacity of S. geographica to invade and establish populations in novel locations. Whilst historic records from the Australian east coast indicate a latitudinal distribution from northern Queensland south to Sydney, observations reported in this paper confirm that its range extends to the southern east coast, an area regarded as an important climate change hot-spot. These records not only represent an important southward shift in range, but are also the most southerly global observations for this tropical taxon. Observations from the Mediterranean and those reported here, were generated by citizen scientists, highlighting the substantial benefit of public engagement in ongoing programs that document, and monitor changes in, marine biodiversity.