Large hollow-bearing trees are key habitat structures in the tropical savannas of northern Australia, as they are in other savannas globally. It is important to understand the relationship between the abundance of tree hollows and the decline of hollow-dependent mammal species. Of the nine mammal species that have declined markedly in northern Australia since European settlement, six are arboreal (including semi-arboreal and climbing species) and all use hollows either by necessity or opportunistically. Many of these species have now contracted to the higher rainfall parts of their range where large trees (and therefore larger hollows) are most abundant. This study is the first of its kind in Australia’s tropical savannas. It aims to quantify hollow abundance to form a foundation for assessing whether there is a causal relationship between arboreal mammal decline and hollow abundance. The key questions it seeks to address are how hollow abundance varies across this environment, the factors that affect that abundance, and how this may affect hollow-dependent species and their management.