Platypus live only in Australia, inhabiting a wide range of flowing and still freshwater bodies from sea level up to an elevation of more than 1600 metres near the top of the Great Dividing Range. The species resides along the eastern and southeastern coast of mainland Australia from the Glenelg River catchment in western Victoria to about as far north as Cooktown in Queensland.
In broad terms, populations still occur in about 80% of the river basins in Victoria, all of the east-flowing river systems and about 80% of the west-flowing systems in New South Wales, and around a third of reliably flowing river basins in Queensland. Predation by salt water crocodiles (Crocodylus porosus) and the severe flooding that often occurs along Australian tropical rivers in the wet season may both potentially contribute to the platypus’s northern distributional limit.
The platypus is widely distributed in Tasmania, occupying lakes, ponds and glacial tarns as well as rivers and streams. The species also occurs in the rivers of King Island, which has been isolated from Victoria and Tasmania by the waters of Bass Strait for 10,000 or more years.
Only a few platypus specimens were ever collected in South Australia. Most originated along the Murray River (to as far downstream as Lake Alexandrina), although some were obtained along the Torrens and Onkaparinga Rivers before 1900. It is generally accepted that platypus no longer occur in the wild on the South Australian mainland, although vagrants moving downstream along the Murray River may occasionally enter the state. An introduced population is found near the South Australian mainland on Kangaroo Island, where animals were released in Flinders Chase National Park between 1928 and 1946. The absence of platypus populations to the north and west of South Australia undoubtedly reflects the rarity of reliable surface water in these areas.
The Australian commonwealth and relevant state governments do not consider the species to be threatened (apart from South Australia, where it is listed as endangered). However, the status of the species was recognised as Near Threatened in The Action Plan for Australian Mammals 2012 (published in 2014) and subsequently listed as such by the IUCN. This listing reflected the fact that there is ample evidence that platypus populations have declined precipitously in many parts of their range.
As an example, a reasonable (though conservative) estimate for the number of platypus occupying the Wimmera River basin in western Victoria at the time of European settlement would have been in the order of 1500 animals. Mark-recapture studies carried out by the Australian Platypus Conservancy confirmed that this had declined to less than 200 animals by the 1990s, with animals mainly found in the Wimmera River upstream of Glenorchy and the Mackenzie River downstream of Lake Wartook.
Habitat degradation, channel sedimentation, use of drum nets that drowned platypus as by-catch, and regulation of natural flows by the Wimmera Mallee Stock and Domestic System (which by the 1980s exported about half of the annual flow of the upper catchment to storage reservoirs) would all have contributed to reduced population size.
Further catastrophic losses occurred in the summer of 2006/2007, when exceptionally dry weather resulted in at least 95% of the Wimmera River channel upstream of Glenorchy drying out for an extended period of time. Along the Mackenzie River, a few animals presumably survived this drought in habitats maintained by a small environmental flow from Lake Wartook, but population size would have been strictly limited by the small amount of stream channel (c. 12 kilometres) available.
Similarly, platypus has not to the best of our knowledge been seen at any location in the neighbouring Avoca River basin since the mid-2000s. In the Bass River catchment in West Gippsland (where platypus appear to have been widespread until at least the 1980s), the most recent reliable sighting dates from 2003.
Factors contributing to the platypus’s vulnerability to predicted longer-term patterns of climate change include the animals’ complete dependence on adequate surface water for survival, their characteristically low population density and low reproductive rate, and the fact that female platypus are likely to be out-competed for food by larger (and more aggressive) males and therefore suffer disproportionately high mortality rates when surface water is severely limited.
Grant, T.R. (1998). The historical and current distribution of the platypus, Ornithorhynchus anatinus, in Australia. Pp. 232-254 in Platypus and Echidnas (edited by M.L. Augee). The Royal Zoological Society of NSW, Sydney.
Woinarski. J C.Z., Burbidge, A.A. and Harrison, P.L. (2014). The Action Plan for Australian Mammals 2012. CSIRO Publishing. Collingwood VIC.