Adelaide's little-used desalination plant will be cranked up as part of a deal to provide relief to drought-ravaged farmers in the eastern states.

The bargain between State and Federal Governments will provide up to 100 billion litres of water for farmers to grow fodder, silage, and pasture — enough to produce 120,000 tonnes of feed for animals.

So how will the deal work?

The one-off deal negotiated between the two governments will see the desalination plant increase production to provide 40 gigalitres — 40 billion litres — of water this financial year and will flow into Adelaide's drinking water supply, which is supplemented by water from the Murray River.

The decision to turn on the desal plant means an equivalent volume of water can be returned to the river and used for the upstream drought relief.

All going well, the desalination plant will provide a further 60 billion litres in 2020-21.

As part of the deal, the Federal Government will also spend $10 million on a "Drought Resilience Fund" for South Australian farmers.

What is the desalination plant doing now?

The decision to build the desalination plant came in 2007, when Adelaide's drinking water supply was under threat at the height of the Millennium Drought.

It was originally designed to provide up to 50 gigalitres of water a year, but a deal between former SA premier Mike Rann's government and Kevin Rudd doubled its output to 100 gigalitres, taking the total cost of the facility to $1.8 billion dollars.

Water ministers have been discussing the future of the Murray-Darling Basin Plan. ( ABC Rural: Brett Worthington )

Since its opening in 2011, the plant has produced just 148 gigalitres in total.

In fact, it switches off during the wettest months, and spends most of the year in "minimum production mode", providing no more than 8 gigalitres of water to Adelaide each year.

So, why isn't it being used now?

Water is still flowing down the Murray. And desalinated water is far more expensive than river water.

In fact, the desal plant's electricity bill was $13.5 million for the 2016/17 financial year, despite it producing only 2 per cent of the state's water supply.

As drought conditions have worsened, the South Australian Government has faced increasing pressure to turn the desalination plant on.

Last year, it agreed to study a proposal to do just that, as part of a separate deal to secure extra water under the Murray Darling Basin Plan.

It's expensive, who pays?

Labor's Susan Close questioned the impact it could have in the event of a dry summer and Greens Senator Sarah Hanson-Young said it was setting a "dangerous precedent" and the cost burden could fall to SA.

However, the Federal Government has agreed to meet all costs associated with increased use of the desalination plant and will provide the extra water to farmers at less than market rates.

The SA Government says there will be no adverse impact on South Australia's water security or water prices.

Once the first 40 gigalitres of desalinated water is delivered, a review will be conducted to assess its effectiveness, water availability in the basin, and South Australia's water security and costs.

Greens Senator Sarah Hanson-Young said it could set a dangerous precedent. ( AAP: Dean Lewins )

The SA Government says the second tranche of water is dependent on that review.

"South Australia survived some of the most extreme effects of the Millennium Drought and fully understand the terrible impacts that drought has on farmers, families, regional communities and the nation," SA Premier Steven Marshall said.