Greens MLC Cate Faehrmann said it was "outrageous" that the document had been kept from the Legislative Council, the state's house of review.

"If it is the best option for the Tamworth community you'd think they'd want proof of this on the front page of the paper," she said.

"The people of Tamworth deserve to know whether upwards of $800 million of taxpayers' money is being spent wisely, and whether it will actually do anything to secure their water supply."

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The project cost, its cost-benefit ratio and who will receive water allocations as a result of the 22-gigalitre new dam, are all currently secret. The government has previously declared the document "will not be made publicly available".

NSW parliament last month issued a call for papers, requiring the government to release the project business case, a document which would contain those details. The release was due on Wednesday last week.

But the state government has refused to do so, claiming the business case is out of scope of the order because it contains cabinet and commercially sensitive information.

Tamworth MP and Water Minister Kevin Anderson told the Leader he had made progress on getting the new Dungowan Dam moving.

"I will ensure this dam is delivered for the people of Tamworth and the Peel Valley," he said.

"I can assure the community that things are moving and we will get this dam, which will sustain water security for town water and agriculture in our area."

Mr Anderson revealed in February that an "overdue" completed business case had been sent to Infrastructure NSW and Infrastructure Australia.

The MP committed to "continue to update the community as we make progress in securing our region's water supply".

Greens MLC Cate Faehrmann, who sponsored the motion demanding the release of the documents, accused the National Party of being "afraid of this business case being released for public scrutiny".

"The government's refusal to comply with this order for papers, which was agreed to by the Legislative Council, is unacceptable," she said.

"The leader of the government has been censured before for failing to produce documents requested by the house and I'll be pursuing this when we sit again in two weeks."

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The dam project was originally budgeted to cost $484 million, split between state and federal governments. It's now widely expected to blow that budget by as much as double.

Ms Faehrmann said even if the documents do have sensitive information, they should have been produced for members of the Legislative Council to review, without being released to the public.

Rather than redacting the documents to remove facts considered commercial-in-confidence, they simply weren't released at all.

The state government has committed to publicly exhibit the project's environmental impact statement, which contains elements of the final business case, before the end of the year.

The final business case is currently with Water Infrastructure NSW and Infrastructure Australia.

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