Taxpayers have been ripped off by the sale of a division of the Water Corporation that has netted WA residents just over $2 million, the Opposition says.

WA Labor water spokesman Dave Kelly said the corporation's construction and engineering division was sold for $10.4 million, but after expenses and taxes the Government was left with about $2 million.

"The Water Corporation paid out $7.4 million in consultant's fees and in redundancies," he said.

"There will be a further tax bill of almost $1 million, leaving precious little for the taxpayer out of the $10.4 million sale price."

Mr Kelly said Water Minster Mia Davies needed to release the business case that supported the privatisation plan, to justify why it was sold in the first place.

"What was in it for the taxpayers to sell this asset for such a small amount of money?" he said.

"And then have to purchase the services of that asset back from the private company."

According to a document filed with the Australian Stock Exchange in August by RCR Tomlinson, the private firm which bought the asset, the Water Corporation has guaranteed $130 million in work over the next three years as part of the purchase.

The acquisition also gives RCR selected bidder status to tender for additional Water Corporation works for two years, with the prospect of this being extended.

Questions again raised over Water Corporation chair's role

In a statement, WA Labor said the deal "looked worse" because Water Corporation chair Eva Skira was also a director of RCR Tomlinson.

Ms Davies last year said Ms Skira only removed herself from the process once the initial decision to make the sale had been made.

Today Ms Davies said the decision to sell the division was not about making a profit, but instead planning for the future.

"It wasn't done on the basis of making a return, it was made around an operational decision by the board, to manage the workflow for the ECS [Engineering and Construction Services] going forward, and how best placed they would be to respond to a changing population growth projection," she said.

"They were either looking at making people redundant, or they were looking at trying to find ongoing employment for a majority of employees which is what happened."

Ms Davies said all information that could be provided had been through questions put in Parliament.

She said the majority of employees had been able to remain employed after the privatisation.