We have investigated the impact of the unprecedented drought, heatwaves and fires of 2019–20 on a threatened arboreal marsupial, the Greater Glider (Petauroides volans), in the Greater Blue Mountains World Heritage Area, an area of over one million hectares. The study involved multiple post-fire surveys in 2020 and 2021 of 16 transects (eight burnt and eight unburnt) for which we had pre-2019 data on Greater Glider numbers. We were unable to find any gliders on two transects burnt at high to extreme severity (100% of eucalypt foliage killed in the canopy and understorey) but Greater Gliders were still present on all six transects burnt at low to moderate severity (44–77% of eucalypt foliage killed), although in significantly lower numbers (mean decline of 43% per transect). Greater Gliders feed on eucalypt leaves and their post-fire survival appears to be dependent on the presence of eucalypt trees with abundant unburnt foliage, which were absent from the more severely burnt transects. Even 17 months after the fires, Greater Gliders were still dependent on these trees, which constituted 36% of trees on low-moderate burnt transects but 77% of trees in which gliders were seen. Gliders were seen feeding on post-fire epicormic regrowth but generally avoided trees with mainly epicormic foliage, possibly because they provided insufficient protection from predators. Greater Gliders were also seriously impacted by the preceding drought and heatwaves, with patchy but significant declines on the eight unburnt transects (mean decline of 34% per transect). We measured the likely impact of the fires on Greater Glider habitat across the World Heritage Area by determining fire severity at a total of 773 precisely located points at which the species had been recorded. An extraordinary 84% of their known sites had been burnt, with 34% burnt at high-extreme severity, 50% burnt at low-moderate severity and only 16% unburnt. Combining these figures with the Greater Glider declines recorded on our transects, we estimate that the Greater Glider population of the World Heritage Area has been reduced by 61% by the extreme events of 2019–20. This is a broad extrapolation from a limited number of sites but it indicates the massive scale of the likely impacts.