Today is Census Night
"The Census is the largest statistical collection undertaken by the Australian Bureau of Statistics (ABS) and one of the most important. There are two broad objectives that underpin the Census. The first of these is to accurately measure the number and key characteristics of people in Australia on Census Night and the dwellings in which they live. The second is to provide timely, high quality and relevant data for small geographic areas and small population groups, to complement the rich but broad level data provided by ABS surveys." (http://www.abs.gov.au/websitedbs/d3310114.nsf/Home/census)

Or see Wikipedia