Most of the 1,061,469 km² basin is flat, low-lying and far inland, and receives little rainfall. The many rivers it contains tend to be long and slow-flowing, and carry a volume of water that is large only by Australian standards.
The basin drains roughly three-quarters of New South Wales (including all of the A.C.T.), half of Victoria, a substantial portion of southern Queensland, and a small part of eastern South Australia. In the drought throughout much of Australia during the early years of the 2000s, many people complained that upriver dams were stopping too much of the water flow in the lower parts of the river system.
In general, the climate is hot and dry in summer and mild in winter. Much of the terrain is semi-arid and nearly all of it is only a few tens of metres above sea level. Typically, tree-lined watercourses meander slowly through mulga or mallee scrub, grasslands or chenopod shrublands. Rainfall is unpredictable and varies from place to place, as well as year to year, but is typically 250 to 300 mm a year (10 to 12 inches).
In the south-east, average temperatures are lower, elevations a little higher, and rainfall more frequent: 500 mm (20 inches) a year is representative, most of it falling in winter and spring. Along the southern and eastern borders of the basin are the inland slopes of the Great Dividing Range. It is here that most of the water in the rivers of the basin originates either as rainfall or, in the case of the Australian Alps which straddle the New South Wales-Victoria border, as winter snowfall. .
Introduced in around 1850, the four varieties of Carp were used to stock up fish dams. Since then they have made their way into the river systems, where they spread quite quickly. These fish are very mobile, as they can travel easily on flood waters and their eggs can be transported by birds.
These fish are a problem because they feed by sucking gravel from the river bed and taking all the edible material off it, before returning the rest to the water. This stirs up all the sediment, reducing the quality of the water.
Of the approximately 13,000 gigalitres of flow in the basin which studies have shown to be divertible, 11,500 gigalitres is removed for irrigation, industrial use, and domestic supply. Agricultural irrigation accounts for about 95% of the water removed, with the growing of rice and cotton being highly controversial among scientists in Australia, owing to their high water use in a region extremely short of water (as much due to exceptionally low run-off coefficients as to low rainfall).
The Murray River is the largest of the basin's many rivers, the Darling River the longest. It is 3370 kilometres from the Queensland headwaters of the Darling to the Coorong in South Australia, where the Murray flows into the sea. The major rivers of the basin are:
In April 2007 the former Australian prime minister John Howard announced that the region was facing an "unprecedentedly dangerous" water shortage and that water might have to be reserved for "critical urban" water supplies. He commented that "We should all pray for rain because the situation for the farmers of Australia in the irrigation area, the Murray-Darling Basin, is critical" amid concerns about the impact of the drought on Australian agriculture.
The Federal Government proposed a $10 billion Commonwealth take-over of the Murray-Darling Basin, arguing that effective management could not be undertaken by competing state governments. While the states of New South Wales, Queensland and South Australia as well as the Australian Capital Territory accepted the proposal, the state of Victoria refused to co-operate, arguing that its irrigators would be disadvantaged. The Victorian Farmers Federation, however, has since backed the Federal Government's plan.
In July the Federal Government said that in the absence of co-operation from all states it would legislate to achieve a federal take-over, but the Victorian Government has declared that it would challenge this in the High Court.
Legislation to create the Murray-Darling Basin Authority was introduced into Federal Parliament and was passed in both the House of Representatives and the Senate in August 2007.
On 26 March 2008, at the Council of Australian Governments meeting, Premier John Brumby indicated that the Victorian government would participate in the program, in return for $1 billion to upgrade irrigation and continue water security for farmers.