Many of the world's terrestrial mammal species are imperilled, but recent extinctions and declines have been most severe in Australia. In particular, arid-dwelling marsupials in a critical weight range (35-5500 g) have declined dramatically following European settlement. In the absence of long-term monitoring, documenting these declines or distribution shifts and their causes often relies on occurrence data from multiple sources. Using atlas records, we compared the distributions of all currently extant marsupials in the critical weight range in Australia's arid Northern Territory pre- and post-1975. For taxa with evidence of range contraction, we evaluated alternative hypotheses to explain this contraction (i.e. competition, predation, productivity, climate) using several techniques to improve our confidence in the results. Despite a substantial increase in the number of mammal records across the study region post-1975, the bilby Macrotis lagotis and desert form of the common brushtail possum Trichosurus vulpecula appear to have contracted in distribution by 25 and 40%, respectively. These changes in distribution were best explained by hypotheses of competition and climate-change, respectively. Macrotis lagotis was more likely to occur on land without a history of cattle grazing and with low rabbit densities, while T. vulpecula has contracted to parts of its distribution that experience cooler maximum temperatures over the hottest months of the year. For five other taxa (including the vulnerable black-footed rock-wallaby Petrogale lateralis) we recorded increases in distribution post-1975, probably reflecting increased survey effort rather than actual range expansion. We conclude that models using multiple-source occurrence data can provide key insights into the patterns and drivers of contemporary species' declines, and represent useful tools for conservation.