Australia is the second driest continent on Earth and freshwater is, therefore, a critical policy concern. Farm dams are ubiquitous and drive AU$17.7 billion of agricultural value, yet there has never been a formal census of Australian dams. In this study, we present a continental-scale assessment on density, distribution, and historical trends of farm dams in each State and Territory of Australia. We estimated that Australia has 1,838,052 dams occupying an area of 5,001 Km^2 and storing 11,922 GL of water. The State of New South Wales recorded the highest number of dams (642,714, 35% of the total) and Victoria the highest overall density (1.67 dams Km^-2). We also estimated that 284,820 dams (15%) remain unreported across Australia, especially in South Australia, Western Australia, and the Northern Territory. Three decades of historical records revealed an ongoing decrease in the rate of farm dam accumulation, from >3% per annum before 2000, to ~1% after 2000, to <0.05% after 2010 – except in the Australian Capital Territory where rates have remained relatively high. To facilitate sharing information with the Government, scientists, managers, and the local community, we developed AusDams.org: a free interactive portal to visualise the distribution of farm dams and generate statistics for any area of Australia. We hope that this work will encourage future research and outreach on the effects of sprawling farm dams on freshwater resources, food security, and the environment.