Hundreds of species rely on tree-hollows for shelter and breeding, however land-clearing has reduced their availability worldwide. While nest-boxes are deployed extensively in hollow-deficient habitats, their thermal value for arboreal marsupials compared to tree-hollows is unclear, particularly in temperate environments. We analysed thermal regimes in nest-box and tree-hollow pairs during summer and winter environmental conditions. Using a biophysical model, we quantified the relative suitability of den-sites for several marsupial species, estimating the impact of microclimates (and ambient conditions) on predicted heat-production and heat-loss. Nest-box temperatures were strongly influenced by ambient temperatures and solar radiation, whereas tree-hollows buffered external temperature fluctuations. On average, nest-boxes reached maximum temperatures 8 °C higher than tree-hollows in summer, and 3 °C higher in winter, with maximum temperatures of 52 °C recorded in nest-boxes, compared to 38 °C in tree-hollows. During summer, estimated heat-loss required by marsupials was 1.5–2.4 times higher in nest-boxes than tree-hollows. Conversely, predicted winter heat-production requirements were slightly lower in nest-boxes (0.95–0.97 of hollow requirements). Our study emphasises the importance of retaining tree-hollows as thermal refuges for hollow-dependent marsupials in temperate zones to reduce thermoregulatory costs during heat-events. Current nest-box designs are likely of limited value during high temperatures and solar radiation loads if they consistently reach temperatures exceeding species upper critical temperatures, however may provide suitable microclimates during winter. With increasing and more prolonged heat-events predicted under climate change, future conservation-management programs should focus on improving nest-box thermal properties to enhance suitability for wildlife.