Drought in Australia is defined as rainfall over a three-month period being in the lowest decile of what has been recorded for that region in the past. This definition takes into account that drought is a relative term and rainfall deficiencies need to be compared to typical rainfall patterns including seasonal variations. Specifically drought in Australia is defined in relation to a rainfall deficiency of pastoral leases and is determined by decile analysis applied to a certain area.
Beginning in the second half of 1991, a very severe drought occurred throughout Queensland which intensified in 1994 and 1995 to become the worst on record.
By October 1994, part of the upper Darling River system had collapsed and the Condamine River had reverted to a series of ponds. Across the state 40% of Queensland was drought declared. From July to August 1995 the drought was further influenced by a strong El Nino weather pattern associated with high temperatures. According to Primary Industries Minister, Ed Casey, "the drought affected region stretched in a 200 km to 300 km wide strip from Stanthorpe to Charters Towers". So few wheat and barley crops survived, about half the usual for that year, that grains had to be imported from other states.
By 1995 the drought had spread to many parts of Australia and by 2003 was recognised as the worst on record. Despite slightly above normal summer/autumn rainfall, in 2006, the late-winter to mid-spring rainfalls failed, resulting in the 2006 annual rainfall being 40—60% below normal over most of Australia south of the Tropic of Capricorn. The average rainfall in the state of South Australia was the lowest since 1900 with only 108.8 millimetres (4.28 in) of rain recorded compared to the normal winter/spring average of 376.6 millimetres (14.83 in). Across Victoria and the Murray-Darling Basin the season was the second driest since 1900. While New South Wales' rainfall was boosted by above normal falls along the north coast of the state, the state's average rainfall for the season was the third lowest since 1900. The situation was exacerbated by temperatures being the highest on record since the 1950s.
Most Australian mainland capital cities faced a major water crisis with less than 50% of water storages remaining. For example, Melbourne had rain up to 90% below the average for September and October 2006, compounding the problem of extremely low rainfall from the preceding winter months. Melbourne had also experienced high temperatures throughout October causing increased evaporation of water in dams and reservoirs, which resulted in their levels falling by around 0.1% a day. As a result of all these factors Melbourne was put on tight water restrictions and as of July 2009, water levels in its dams were at a mere 27% of capacity. Brisbane water levels reduced to under 20% of capacity, having had no substantial inflow for five years.
Agricultural production was severely affected. Australia's cotton production had dropped, with the smallest area planted in 20 years, a 66% reduction compared to five years earlier which was considered a "normal" year. The crop had been half its usual size for three of the previous five years. Water use by the industry fell by 37% between 2000/01 and 2004/05, due mainly to the drought. In the order of 20 cotton communities and 10,000 people directly employed by the cotton industry are impacted by the drought. The main areas affected were in New South Wales: Menindee where the area under production has reduced by 100%, Bourke has reduced the area under production by 99%, Walgett has reduced the area under production by 95%, the Macquarie River has reduced the area under production by 74% and the Gwydir River has reduced the area under production by 60%. In Queensland the worse affected areas are Biloela which has reduced the area under production by 100%, at Dirranbandi there has been a 91% reduction, Central Highlands has reduced the area under production by 82% and Darling Downs has reduced the area under production by 78%. Bourke has had adequate water for only one cotton crop in the last five years. Stock feed was also becoming scarce and farmers are finding it difficult to feed cattle and sheep. Dairy producers were hit particularly hard by the drought with 2004 a particularly bleak year in the sector, as a drought-caused drop in production sent revenue in the industry down by 4.5%.
Australia had previously relied solely on water from dams for agriculture and consumption. The drought changed the way Australia treated its water resources. Because of the long-term effects of the drought now showing, many state governments attempted to "drought-proof" their states with more permanent solutions such as grey-water water-recycling, government rebates for home-owners to install water tanks, and tougher restrictions on industries.
The citizens of Toowoomba voted on, and rejected, a referendum on using recycled sewage water. As a result, no recycled sewage was added to Toowoomba's drinking supply. Brisbane organised to be supplied via larger dams, a pipeline and possibly also recycling. A desalination project was initiated on the Gold Coast, Queensland.
Plans for a desalination project in Sydney were temporarily halted in 2005 after public opposition and the discovery of new underground aquifers. By late 2006, however, with Sydney's water storages plunging to their lowest levels since the 1950s - around 33% of capacity - the authorities decided to reinstate the project. A $1.8 billion desalination plant was then constructed at Kurnell, in southern Sydney, opening in the summer of 2009-10.
In November 2006 Perth completed a seawater desalination plant that will supply the city with 17% of its needs. Likewise, the Victorian Government also began building a $3.1 Billion 150Gl (gigalitre) desalination plant, one of the world's largest. When completed in 2011, it would be capable of supplying up to a third of Melbourne's water needs.
In South Australia work on a small pilot desalination plant at Port Stanvac, costing $10 million and with a capacity of 100,000 litres per day, commenced in January 2008, and was completed on 4 August 2008. In 2007 the Federal Government pledged to contribute funds and construction began on a $1.1 Billion 50Gl (gigalitres) desalination plant.
In June 2009 the South Australian Government announced that the plant's annual output was to be doubled from 50Gl to 100Gl, approximately 270Ml (megalitres) per day, providing up to 50% of Adelaide's domestic water supply.
In early 2007, senior weather forecasters predicted that the drought would ease along the east coast with a return to average rainfall from late February 2007. Forecasters believed that the El Niño effect that had been driving the drought since 2006 had ended.
The Prime Minister at the time, John Howard, announced on 19 April 2007 that unless there was substantial rain in the next six weeks no water would be allocated to irrigators in the Murray-Darling basin for the coming year. The result of this would have directly affected the 50,000 farmers and the economy. Electricity shortages may also have occurred if the Snowy Mountains Scheme had been forced to shut down its hydroelectric generators.
Heavy rainfall in June and July, particularly in coastal regions of New South Wales and in Victoria's Gippsland region, together with tentative forecasts of a La Niña event, brought hope that the drought may have ended. In August 2007, the Darling River flowed again after nearly a year of no flows, however, the Murray-Darling Basin experienced their seventh consecutive year of below-average rain and inflows into the Basin during the winter of 2007 were still amongst the lowest on record, though marginally better than those of the winter of 2006 which had been the driest on record.
The drought in Sydney eased around April 2008 and Sydney's main water catchments reached 65 percent, 25 per cent fuller than they were at the same time the previous year. However Victoria remained drought affected, with Melbourne's water supplies dipping to around 30% by July 2008. While drought conditions eased in parts of South Australia, the fertile South-East of the state received below average rainfall and drought conditions persisted. In Tasmania drought conditions worsened in 2008, with many areas reporting no significant rainfall for three years.
In June 2008 it became known that an expert panel had warned of long term, maybe irreversible, severe ecological damage for the whole Murray-Darling basin if it did not receive sufficient water by October of that year. The Bureau of Meteorology's head of climate analysis, David Jones, released statistics showing that in 2007 South Australia, NSW, Victoria, the ACT and the Murray-Darling Basin all set temperature records by a very large margin. It was the 11th year in a row that the Murray-Darling Basin had experienced above average temperatures while for Australia as a whole, 2007's mean temperature resulted in the year being the nation's sixth-warmest year since records have been kept. Jones warned that "There is absolutely no debate that Australia is warming... it may be time to stop describing south-eastern Australia as gripped by drought and instead accept the extreme dry as permanent."
In 2009, drought conditions in South East Australia continued, after one of the driest summers for the region. Melbourne had had Stage 3a water restrictions from 1 April 2007, and narrowly avoided Stage 4 restrictions, with the minimum storage level of around 25.8% remaining above the threshold of 25% for enacting Stage 4. This led to the construction of the North-South Pipeline which would divert water out of the Goulburn River and supply it to Melbourne to meet water demands. Many towns in Victoria were close to running out of water, with some of the few Victorian towns without water restrictions being in the East Gippsland water area, where reservoir levels were above 80%.
The very low flow to the Lower River Murray in South Australia (over Lock 1) resulted in the lowest water levels in over 90 years of records. The lowest water levels during the extreme low flow period were reached in April 2009 and represented a 64% and 73% reduction in the volume of Lakes Alexandrina and Albert respectively. The low water levels and inflows meant there was no outflow from the lake system during the extreme low flow period. During this period the lake levels fell below mean sea level (approximately +0.2 m AHD) downstream of the barrages, reversing the usual positive hydraulic gradient from the lake to the sea. The seawater intrusion, lack of flushing, evapoconcentration and increased resuspension resulted in severe water quality impacts
Exposure and oxidation of acid sulfate soils due to falling water levels from 2007-2009 in the Lower River Murray and Lower Lakes also resulted in acidification of soils, lake and ground water. Large scale engineering interventions were undertaken to prevent further acidification, including construction of a bund and pumping of water to prevent exposure and acidification of Lake Albert. Management of acidification in the Lower Lakes was also undertaken using aerial limestone dosing.
The 2010 Victorian storms in March did little to help Melbourne's storage levels, but steady winter rains, and the 2010 Victorian floods in September, led to storage levels remaining above around 32.7%, rising to over 46% in September and 51% by late November. 2010 saw Australia officially record its wettest spring on record due to a moderate to strong La Nina developing over the region. Water restrictions were reduced to stage 3 on 2 April, and stage 2 on 1 September. Melbourne also recorded its average annual rainfall in 2010 for the first time since 1996, and its wettest spring since 1993. By 2011, Victoria was drought free for the first time in almost 15 years and was instead gripped by very wet conditions and severe flooding.
The drought in Queensland had mostly eased with Brisbane recording very heavy rain in May 2009, and premier Anna Bligh announcing that South East Queensland was no longer experiencing drought. Brisbane's dams were now at full capacity with the state in general experiencing its wettest spring on record. The end of the drought and heavy spring rains in Queensland culminated with devastating floods in December 2010 and January 2011.
The drought in New South Wales had also eased. In the beginning of 2010 the percentage of the state in drought was close to 70%. By December 2010, the entire state was officially out of drought, with the entire state recording its wettest spring on record. Several rivers, including rivers in the outback had flooded several times, and many dams were overflowing, including the Burrendong, Burrinjuck and Pindari Dams. Canberra's dams were above 90% capacity.
Despite Western Australia experiencing its fifth wettest spring on record, drought in the South West, Gascoyne and Pilbara regions of Western Australia's intensified, with the regions experiencing their driest year on record. Perth's dams registered their lowest inflows on record with the city itself recording its third-driest year on record, along with the hottest spring on record.
In South Australia, only two regions in the Riverland remained in drought. Heavy rains elsewhere led to bumper harvests over much of the state, this in turn led to the largest mouse plague since 1993 across parts of South Australia, West Australia and Victoria. While some farmers tried to replant, in some areas many gave up as millions of mice covered their fields. Farmers often characterised the plague as being worse than the drought.
A large blackwater (low dissolved oxygen) event occurred across a large area of the River Murray during high flows in 2011. Organic carbon (dead plant material), that had been retained in the landscape during the drought, was mobilised into the river system and the breakdown of this consumed dissolved oxygen.
On 27 April 2012, Agriculture Minister Joe Ludwig stated that the two final areas in Australia receiving Federal "exceptional circumstances" drought support, Bundarra and Eurobodalla in New South Wales, would cease being eligible the following week. The Federal Government had provided $4.5 billion in drought assistance since 2001. The related move to end the exceptional circumstances interest rate subsidy program was criticised as premature by the NSW Farmers Association and National Farmers' Federation.
Drought policy research conducted by the Australian Bureau of Agricultural and Resource Economics and Sciences stated that, "there has been a greater emphasis on climate risk management by the Australian Government. This initiative focuses on enhancing the ability, preparedness and responsibility of farmers to manage climate risks".
However, the environmental legacy of the drought persists in parts of the system. In particular, as of 2014 Lake Albert salinity is still elevated after 4 years post-drought and acidity persists in soils and groundwater.