ADELAIDE'S mothballed desalination plant was not expected to have to run at full capacity before 2029 - and up to 2038 if the state remained only moderately dry.

SA Water last week revealed the $1.8 billion plant, funded largely through huge rises in household water bills, will be mothballed.

Two major studies considered by Cabinet amid the 2009 water scare, released at The Advertiser's request, reveal for the first time the full scale of the dilemma which confronted the state.

One study advocates that "demand management", including greater in-home efficiency, be prioritised over a desal expansion.

It was also possible that a combination of permanent water restrictions, demand reduction and savings from new technologies could have been used instead of doubling the plant's capacity.

Then-premier Mike Rann had already buckled to Opposition demands for a 50 gigalitre desalination plant, but water remained a major political issue and a state election was just 10 months away.

A KPMG consultancy report from June 2009 measured expected demand in the next 40 years, with population growth, under moderate and extreme dry scenarios.

It found Adelaide's water demand would outstrip supply as soon as 2013 with ongoing drought and a 50GL plant.

However, in moderate dry conditions full output from 100GL plant would not be needed until 2038.

If SA endured extreme dry years, the 100GL plant would max out in 2029.

Additional water to meet demand would then be delivered by buying more Murray water and "comprehensive demand management".

The warnings cast new light on the $450 million decision to double the size of the plant.

The doubling has cost SA households $222 million and the Federal Government paid $228 million.

The biggest charge of the desal project was setting up the basic 50GL plant infrastructure, which cost SA $1.4 billion and was backed by $100 million from then-prime minister Kevin Rudd.

A separate WorleyParsons analysis from 2009 examined the measures to bridge the gap between supply and demand once the 50GL plant was superseded.

It found "Demand management, particularly when associated with active public education efforts, consistently delivers the best overall environmental, social and economic return for society under all future conditions tested".

It finds desalination is the next-best option, but the "only large-scale option capable of meeting the shortfall".

However, demand management would only deliver about 25GL per year, while the desal plant gives an extra 50GL.

That indicates the Government could have used water restrictions and other demand reduction methods to delay the doubling until as late as 2020 if the state only experienced moderate dry years. However, it is likely the cost of doubling at that point would have been far higher.

The State Government argues the expansion was a necessary "insurance" policy for a dry state.

It has also argued long-term water security is critical for social and business confidence.

However, population forecasts used at the time have not been realised in the years since and are expected to fall even further short with the indefinite delay to the Olympic Dam expansion.

Short-term climate forecasts have also been proved inaccurate after the drought broke in 2010.

Opposition Leader Isobel Redmond has called the doubling expensive and wasteful and says households are paying for Government mistakes.

The WorleyParsons study also examined storm-water recycling and expanding Mount Lofty Ranges storages but ruled them out on cost and environmental grounds.

Water and the River Murray Minister Paul Caica said he stood by the doubling decision.

"The Government was determined not to build the water equivalent of a one-way freeway which did not have the capacity to meet our future full water security needs," he said.

"The decision to expand the capacity of the desalination plant in the 'first build' avoided the costly exercise that was being undertaken at the same time in Perth the construction of a second desalination plant because their first was insufficient."