Demography is the study of statistics of human populations. This includes the investigation of births, deaths, income, disease, growth, density and other related areas.
When looking for demographic information focus on the type of information you need to find, for general information on Australia's population use the Australian Bureau of Statistics you can also gather information from other government agencies or independent bodies. For international information you can gather statistics from the UN and other resources.
The Methods and Materials of Demography by 
The Demography and Epidemiology of Human Health and Aging by