The Great Artesian Basin (GAB) is one of the largest artesian groundwater basins in the world. It underlies approximately one-fifth of Australia and extends beneath arid and semi-arid regions of Queensland, New South Wales, South Australia and the Northern Territory, stretching from the Great Dividing Range to the Lake Eyre depression. The Basin covers a total area of over 1 711 000 square km and it has an estimated total water storage of 64 900 million megalitres (a megalitre is one million litres and is equivalent to about half the water in an Olympic swimming pool).

The Great Artesian Basin was formed between 100 and 250 million years ago and consists of alternating layers of waterbearing (permeable) sandstone aquifers and non-waterbearing (impermeable) siltstones and mudstones. The thickness of this sequence varies from less than 100 metres on the Basin extremities to over 3 000 metres in the deeper parts of the Basin. Individual bore depths vary up to 2 000 metres with the average being 500 metres. Some of the sandstone sequences contain oil and gas where conditions are suitable.

Groundwater in the Basin flows generally westward to the south-west over most of the Basin but to the north-west and north in the northern section. The rate at which water flows through the sandstones varies between one and five metres per year. Recharge by infiltration of rainfall into the outcropping sandstone aquifers occurs mainly along the eastern margins of the Basin, more specifically along the western slopes of the Great Dividing Range. Natural discharge occurs mainly from mound springs in the south-western area. Mound springs are natural outlets of the artesian aquifers from which groundwater flows to the surface. Dating of the artesian waters has given ages of almost 2 million years for the oldest waters, which occur in the south-western area of the Basin. Water quality in the main aquifers is generally good with Total Dissolved Solids (TDS) varying between 500 to 1 500 milligrams/litre, although pH may be very high, and high sodium levels make it generally unsuitable for irrigation. Water temperatures vary from 30 degrees C in the shallower areas to over 100 degrees C in the deeper areas.

Many bores initially flowed at rates of over 10 megalitres per day (ML/d). However, the majority of flows are now between 0.01 and 6 ML/d. Total flow from the Basin reached a peak of over 2 000 ML/d around 1915, from approximately 1 500 bores. Since then, artesian pressure and water discharge rates have declined, while the number of bores has increased. The total flow from the basin during 2000 was in the order of 1500 megalitres per day.