Platypus live only in Australia, inhabiting a diverse array of reliable freshwater habitatss from sea level to an elevation of more than 1600 metres near the top of the Australian Alps (APC, unpub. records). Their current range (marked in blue below) extends along the east and southeast coast of mainland Australia from western Victoria to about as far north as Cooktown in Queensland, and also includes Tasmania and King Island. An introduced population is found offshore on Kangaroo Island (marked in pink below), where platypus were released in Flinders Chase National Park in 1928, 1941 and 1946 (Copley 1995). No subspecies are currently recognised anywhere in the range (Woinarski et al. 2014).
Map adapted from R. Strahan and S. van Dyck. (2008). The Mammals of Australia, 3rd edition. (New Holland: Sydney).
In broad terms, the species is known to inhabit about 80% of the river basins in Victoria, all east-flowing river systems and about 80% of west-flowing systems in New South Wales, and around a third of river basins containing reliably flowing water bodies in Queensland (Grant 1992). Predation by salt water crocodiles and severe flooding in the wet season may both contribute to the platypus’s northern distributional limit in Queensland (Grant 1992). The species is believed to be widely distributed across most of Tasmania, occupying lakes, ponds and glacial tarns as well as rivers and streams. A population is also found on King Island, which has been isolated from Victoria and Tasmania for at least 10,000 years by the waters of Bass Strait (Furlan et al. 2012).
Less than 20 authenticated platypus specimens are known to have been collected in South Australia, all dating from the early years following European settlement. Most originated along the Murray River (to as far downstream as Lake Alexandrina), though some were obtained along the Torrens and Onkaparinga Rivers before 1900 (Grant 1992). The absence of platypus populations in the Northern Territory and Western Australia plausibly reflects the widespread occurrence of crocodiles in the north and the pervasive dryness of the landscape elsewhere.
The platypus’s overall distribution appears to have remained broadly unchanged since pre-European times, though many populations are believed to have declined due to habitat degradation and altered flow regimes (Grant and Temple-Smith 2003). The best documented range contractions have been in river systems altered as an outcome of pervasive urban development in and near Sydney (Grant 1998), Melbourne (Serena and Pettigrove 2005), and Brisbane (Brunt et al. 2021). In rural environments, some already small Victorian platypus populations apparently became extinct in the early 2000s during the prolonged and very severe Millennium Drought. For example, this species has not (to the best of our knowledge) been seen at any location in the Avoca River basin in western Victoria since 2003 (APC unpub. records). In the Bass River catchment in southern Victoria (where platypus seem to have been widespread until at least the 1980s), the most recent reliable sighting also dates from 2003 (APC unpub. records).
Similarly, Serena and Williams (2007) estimated that a reasonable estimate of platypus population size in the Wimmera River basin in western Victoria at the time of European settlement would have been around 1500 animals. By the 1990s this had declined to less than 200 animals, mainly occupying the Wimmera River upstream of Glenorchy township and the Mackenzie River downstream of Lake Wartook. Habitat degradation, channel sedimentation, use of fishing nets and traps in which platypus drowned as bycatch, and regulation of natural flows by the Wimmera Mallee Stock and Domestic System (which by the 1980s exported about half of the upper catchment’s annual flow to storage reservoirs) all contributed to reduced population size. Further catastrophic population losses occurred in the summer of 2006/07, when severe drought caused at least 95% of the Wimmera River upstream of Glenorchy to dry out for months. A few animals survived in the Mackenzie River due to water released from Lake Wartook, but population size was very much limited by the small amount of habitat available (around 12 kilometres of channel). However, the good news is that this population has since grown in size and expanded its range along the Mackenzie River, and platypus are also starting to be seen elsewhere in the Wimmera system.
LITERATURE CITED
Brunt T, Cecil M, Griffiths J, Adams-Hosking C and Murray P (2021) Where are the platypuses (Ornithorhynchus anatinus) now? A snapshot in time of their distributon in the Greater Brisbane region. Australian Mammalogy 43, 368-372.
Copley PB (1995) Translocations of native vertebrates in South Australia: a review. Pp. 35-42 in Reintroduction Biology of AUstralian and New Zealand Fauna (M. Serena, ed.) Surrey Beatty & Sons: Chipping Norton NSW.
Furlan E, Stoklosa J, Griffiths J, Gust N, Ellis R, Huggins RM and Weeks AR (2012) Small population size and extremely low levels of genetic diversity in island ppulations of the platypus, Ornithorhynchus anatinus. Ecology and Evolution 2, 844-857,
Grant TR (1992) Historical and current distribution of the platypus, Ornithorhynchus anatinus, in Australia. Pp. 232-254 in Platypus and Echidnas (ML Augee, ed.) Royal Zoological Society of New South Wales: Mosman NSW.
Grant TR (1998) Current and historical occurrence of platypuses, Ornithorhynchus anatinus, around Sydney. Australian Mammalogy 20, 257-266.
Grant TR and Temple-Smith PD (2003) Conservation of the playtpus, Ornithorhynchus anatinus: threats and challenges. Aquatic Ecosystem Health & Management 6, 5-18.
Serena M and Pettrigrove V (2005) Relationship of sediment toxicants and water quality to the distribution of platypus populations in urban streams. Freshwater Science 24, 679-689.
Serena M and Williams GA (2007) Wimmera platypus conservation manual. Report to Wimmera Catchment Management Authority. Australian Platypus Conservancy: Wiseleigh VIC.
Woinarski JCZ, Burbidge AA and Harrison PL (2014) The Action Plan for Australian Mammals 2012. CSIRO Publishing: Collingwood VIC.