Arguments about the extinction of Australia's megafauna have largely rested on anthropogenic factors consequent upon the arrival of humans there, and have lacked any appreciation of the possibilities of climate/environmental changes taking place during the late Quaternary. Moreover, the status of the megafauna at the extinction and in the period leading up to it has largely been ignored. This article assesses the species that existed during the late Quaternary, their continental dispersal, the likely impact of negative climate change during that time and the effect this had on their demography and variety. These factors are discussed together with a synthesis of present data regarding Australia's mega 2004: fauna demography and which species may have reached the extinction threshold. One interpretation of the data suggests a mid-late Quaternary process of demographic fragmentation, disjunction and fluctuation, a restricted continental distribution among a diminishing group and a limited and reducing species variety due to climate and environmental change. It is argued that increasing continental aridity during the mid-late Quaternary was a forcing mechanism behind species distribution, changes to that distribution and population reduction through episodic but widespread drought and vegetation change. This resulted in alteration of the biogeographic status of the megafauna, with increasing stress on and reduction of the population as a whole. In particular, it changed population composition and reduced species variety and overall population size by the beginning of the last glaciation, such that at the time of human entry the population had reached a precarious stage vulnerable to any level of subsequent anthropogenic activity with the arrival of humans in Australia.