In late 19th century western ringtail possums probably occurred from the Perth area down to the Kalgan River region east of Albany. (De Tores, 2000)
Monitoring was highly insufficient for all management zones and survey data is lacking across the range. 
Historically, the Upper Warren inland colony was the largest population with some estimates as high as 100.000 animals. (WRP Recovery Plan, 2017) In the 1990s the population size was still an estimated 30,000 to 50,000 animals. (Jones, 2016)

Local declines occurred throughout the decades and were most extensive in the northern and dryer areas of their range, with the steepest decline (up to 99%) in the Upper Warren region probably form 2004 onwards. (Jones, 2016)
The first local extinctions could have occurred as early as the 1920s. However, by approximately 1980 about 80% of the original range was no longer occupied (Jones, 2004) and at the end of the 20th century the area around Busselton with its coastal Agonis flexuosa woodlands carried most likely the largest population.

Patchy decline over a wide geographic area is ongoing and the area of occupancy is further contracting. (Woinarski et al, 2014)
As surveys in the Swan Coastal Plain were mostly conducted as a condition for development approvals and not centrally collated and as there is no comprehensive summary available listing all clearings of native habitat that did not necessitate clearing approval, we do not even know what we have lost. (EPA Annual Report  2015-2016)
The extent of small populations in remnant vegetation patches is also unknown. 

What we know is that the species now occurs naturally in patchy distribution in coastal areas from Binningup/Myalup to the Leeuwin Naturaliste National Park (Augusta, particularly East Augusta), on the south coast from approximately Walpole to east of Albany and inland in the lower Collie River Valley, at Harvey and surrounding forest blocks near Manjimup. (De Tores, 2000, WRP Recovery Plan, 2017) Upper Warren (Perup, Kingston National Park) numbers have declined to almost undetectable levels. (Wayne et al, 2012)
A small isolated population near Dawesville resulted from translocations to Yalgorup National Park. Remaining animals in the Park might have perished in the recent Yarloop fires. Populations translocated to Lane-Poole conservation Park near Dwellingup and to Leschenault Conservation Park near Bunbury have not persisted and numbers in Perup Nature Reserve seem unsustainably low. A small population translocated to Karakamia (AWC) had established successfully and persisted for several possum generations without expanding but has been lost in the recent past.
Some records listed ringtails near Northcliffe, however they might have also perished in the recent fires.
A breeding colony has also been reported from the Mt Barker area. (Burbidge, 2016)