In a fiery estimates hearing on Wednesday, NSW Water Minister Kevin Anderson was pressed over and over by Labor MLC Rose Jackson about the government's decision to keep the project's business case a secret.

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Ms Jackson said the community couldn't make an informed opinion because the business case, which includes the cost of the project, water allocations and how it will be paid for, remains confidential.

"Why are you keeping it cabinet-in-confidence? What's secret in there? What can't we know?" she said.

"You're lucky, you're privileged, you get to see all the information.

"Good for you, minister."

Mr Anderson said the community would see the information once the project's environmental impact statement (EIS) is released.

"It's part of cabinet process, Ms Jackson," he said.

"The business case contains significant commercial-in-confidence clauses and it remains a document in cabinet."

But, Ms Jackson wasn't convinced - arguing the EIS only happens after the business case is assessed and the decision is final.

"So that's already happened, and then [it's like], 'Oh by the way, community, sorry we've already made all these decisions based on information that you don't see and now, now we will consult you," she said.

"Shouldn't it be the other way around?"

Ms Jackson said that under the National Water Initiative, the cost of water infrastructure is largely recovered through higher water rates.

It means Tamworth residents could shoulder some of the cost, with a WaterNSW feasibility study estimating the community would be willing to pay a minimum of an extra $552 per household and business each year for water.

Department of Planning and Environment chief executive Jim Bentley said the cost would be determined by the Independent Pricing and Regulatory Tribunal.

More than two years after it was announced, the cost, cost-benefit ratio and who will receive water allocations from the 22.5-gigalitre dam remains secret.

It's not expected to be released until after the EIS is published, despite the business case being completed in February.

The government has previously said the business case "will not be made publicly available".

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