Seagrass meadows provide valuable ecosystem services and recent losses of significant areas have emphasised the need to carefully manage the remaining meadows in temperate Australia. Management of ecosystems is generally applied at the species level, however, there is uncertainty on the number of species in the most common seagrass genus in this region, Posidonia. This research aims to resolve the number of species of Posidonia, their distribution, and defining characteristics. The use of traditional morphological characteristics combined with a modern molecular approach will place this study in a uniquely authoritative position to resolve the taxonomy. It has significance from a theoretical perspective but also for the conservation of Australia’s seagrass flora.