The increasing popularity of snow-based recreation activities and the development of ski resorts and associated infrastructure have the potential to affect adversely small mammal fauna that over-winter in the subnivean space. We investigated the effects of human activities on the maintenance of the subnivean space, which is critical to the over-winter survival of small terrestrial mammals in Kosciuszko National Park, south-eastern Australia. The creation of ski pistes, surface ski lifts and over-snow routes involves compression of the snowpack and resulted in small or absent subnivean spaces (average 1.2 cm) and high snow cover densities (generally over 0.5 g cm'3 and 0.35 g cm'3 respectively). By contrast, the subnivean spaces associated with unmodified snow cover averaged 8'20 cm depending on vegetation type. The density of unmodified snowpack was less than 0.35 g cm'3 in June but increased throughout the season to levels comparable to those of compressed snow. When the snowpack was experimentally compressed at 22 sites, destroying the subnivean space, detections of two small mammal species (Rattus fuscipes and Antechinus swainsonii) significantly (p < 0.0001) declined by 75'80%. These species remain active below the snow throughout the winter and depend on the presence of an adequate subnivean space. The removal of vegetation by fire significantly (p < 0.0001) reduced the size of the subnivean space regardless of habitat type. Vegetation clearing occurs as part of ground preparation prior to establishing ski runs. Supergrooming, in which surface soil is also disturbed, is likely to have similar (if not more extreme) effects. Nival areas used for snow-based recreation should be managed to minimise negative effects on subnivean fauna, by maintaining natural features associated with subnivean space formation (dense shrubs, boulders and/or microtopography) and confining developments to areas where these features are not present.