Abstract: This exploratory paper seeks to develop a deeper understanding of the phenomenon of urban density and its effects on the sustainability of urban development, functionality and management of Australian cities. The paper explores different attributes of urban density and presents a conceptual framework to explore the relationship of these to sustainability criteria. The work presented in the paper is confined to an exploration of urban density demographic and urban residential development. Other dimensions of density will be explored in future research. The paper presents simple time series measurements of change in urban density for selected Australian cities drawing upon ABS and satellite imagery data. It shows since 1990 the urban areas of Australia’s largest cities (Sydney) are growing at over 2.4% per annum, while the population growth rate is around 1.2%. The population densities of Australian cities have been falling at about 0.8% per annum. If these trends continue, the implications on the sustainability and development of Australian cities could be significant. The paper examines some environmental impacts of urban density on the sustainability of urban systems. It concludes with a brief discussion on the challenges to achieving sustainable urban development in Australian cities.