SA’s Auditor-General has found the power bill for Adelaide's desalination plant may not be worth the amount of water it produces.

Electricity for the plant cost $13.5 million last financial year, despite it producing just 2 per cent of the state's water supply.

But the same report reveals the plant's operational costs have dropped each year since the 2013-14 financial year.

The 2017 Auditor-General's Report revealed that in 2013-14, expenditure for operational power was $42 million, but that decreased to $22 million in 2014-15, $17 million in 2015-16 and $13.5 million in 2016-17.

Opposition Environment spokesperson David Spiers said SA Water's desalination plant at Port Stanvac is too big and too costly.

"Adelaide's desalination plant has been shown to be double the size that it ever needed to be," he said.

"The productivity commission showed that a couple of years ago.

"It's barely running yet costing tens of millions of dollars."

The 2017 Auditor-General's Report revealed that in 2013-14, expenditure for operational power was $42 million.

It decreased to $22 million in 2014-15, $17 million in 2015-16 and to $13.5 million in 2016-17.

SA Water Minister Ian Hunter said the Opposition is once again taking their “classic approach to privatisation”.

He said it is common practice to attack state-owned utilities for inefficiency, and then sell them “to their mates”.

“I don't know where the Liberals got their figures from. If you actually look at the Auditor-General's report, it will tell you the costs for electricity [at the desalination plant] have been falling in recent years,” Mr Hunter said.

The desal plant was built in response to the Millennium Drought of 2001 to 2009, but it has been run at minimal capacity of about 10 per cent ever since.

Mr Hunter said the average SA Water household water bill was down by $171 in the past three years.

“They've also been able to provide an extra 8 per cent opening water allocation in dry years to our Riverland irrigators, at the same time as providing Adelaide with an independent source of water for our security during a dry period,” he said.

The SA Government has plans to provide an extra 50GL of water to holders of irrigation and equivalent licenses when water allocations are less than 100 per cent.

It intends to do this by reducing Adelaide's allocation of River Murray water from 150GL to 100GL, while drawing more water from the desalination plant if necessary.