Posidonia australis is endemic to the temperate waters of the south-eastern, southern and south-western coasts of Australia. It occurs in 20 estuaries along the east coast of New South Wales from Wallis Lake to Twofold Bay near the New South Wales/Victorian border. Six locations within New South Wales (Port Hacking, Botany Bay, Sydney Harbour, Pittwater, Brisbane Waters and Lake Macquarie) have suffered significant population decline and have been listed as endangered populations under the Threatened Species Schedules of the Fisheries Management Act 1994.
Posidonia australis is a marine angiosperm rather than a true seaweed and is one of eight Australian endemic species in the family Posidoniaceace. Posidonia australis has large bright green strap-like, fast growing leaves of about 30-60 cm in length and 6-14 mm wide with rounded ends. Posidonia australis can be found in large meadows at shallow depths in estuaries, coastal lakes and sheltered coastal waters.