Culex molestus was probably introduced into Australia in the 1940s and represents a potentially important nuisance-biting pest and vector of disease-causing pathogens in urban areas. The aims of this study were to review the literature to determine the current and historical distribution of Cx. molestus in Australia, analyse the genetic similarity of specimens collected from various locations in Australia with reference to specimens from North America, Asia and Europe, and document the seasonal abundance of this mosquito in the Sydney region. Results showed that Cx. molestus is common in southern Australia, but there was no evidence that this mosquito is found north of latitude 28.17°S. Molecular analysis indicated that specimens from various locations throughout Australia shared strong genetic similarity and that it was most likely introduced from Asia, possibly through multiple introductions over the past 70 years. Analysis of the seasonal abundance of Cx. molestus indicated that the species does not display diapause during the cooler months. Consideration should be given to the unique biology and ecology of this species when assessing the public health risk and the surveillance methods required in the management of Cx. molestus within urban areas of Australia.