A drainage basin is an extent of land where water from rain and melting snow or ice drains downhill into a body of water, such as a river, lake, reservoir, estuary, wetland, sea or ocean. The drainage basin includes both the streams and rivers that convey the water as well as the land surfaces from which water drains into those channels, and is separated from adjacent basins by a drainage divide.

Major drainage basins are coded by hierarchy within the National Catchment Boundaries (NCB) dataset, with primary drainage basins attributed Level 1 and smaller river catchment subdivisions attributed Level 2. Beyond that, minor river and creek watersheds are ranked by the Pfafstetter Coding System. This article deals with surface water rather than groundwater basins, such as the Great Artesian Basin.

Australia has twelve distinguished NCB Level 1 drainage divisions^[1] or thirteen^[2] after splitting the South East Coast division at the New South Wales–Victoria border as defined by the Australian Water Resources Assessment 2012, a hydrological survey conducted by the Bureau of Meteorology.^[3] Runoff from these divisions generally flows into the Pacific Ocean, the Indian Ocean, the Southern Ocean and Lake Eyre.