Cate Faehrmann said the half-a-billion dollar project will get a grilling before the new inquiry.

But despite criticising government's secrecy and lack of business case for the scheme, she was quick to insist members would remain objective about the dam.

READ MORE:

"It's not like this is an anti-dams inquiry," she said,

"But it will be looking very thoroughly at whether dams are the right solution now. What we know is that dams won't make it rain.

"Obviously [the dam projects] were an election commitment by the National party. Usually election commitments aren't based on a thorough cost-benefit analysis and that's what the government should have done before hitting go on these projects."

When opened in 2025 the $480 million, 22.5 gigalitre storage dam will be the first built in NSW in nearly 30 years.

The massive dam project was fast-tracked under new planning rules.

But Ms Faehrmann said the project, which won't have a public business case until eight months after construction starts, was approved without a genuine consideration of alternatives to improve water security.

"For the government to be allocating half-a-billion-dollars towards an infrastructure project without looking at alternatives, without having on the table the impacts to all communities - that classic cost-benefit analysis if you like - it's extraordinary.

"It beggars belief that this hasn't been done."

She echoed comments made by Tamworth Mayor Col Murray, that the new dam could be just a waste of half-a-billion-dollars on not much, if government won't change water rules like the Peel Valley Water Sharing Plan.

Leaders from the Commonwealth and State governments announced in October last year they would split the cost of the project 50/50.

Want more local news? Subscribe to the Leader to read it here first

Premier Gladys Berejikilian said she wanted the Dungowan Dam finished by 2025, with construction slated to begin by the end of 2020.

A key function of the inquiry will be to look at the rationale of the project, Ms Faehrmann said.

"That doesn't just mean the financial rationale - the business case, if you like - but also really what the politics are behind these decisions.

"Who has influenced who in terms of these decisions.

"They're very controversial, there are a lot of stakeholders in fact all across the state that are extremely concerned at how these projects have been fast-tracked and who is to benefit."

Have your say, send a letter to the editor.

Ms Faehrmann said the committee will remain objective about the merits of the project.

The inquiry, though led by a Greens politician, will also include MLCs from the Liberals, Nationals and Labor parties and the Animal Justice Party representative.

The inquiry will investigate whether changes to water sharing arrangements, or rules about recycled water could achieve the same outcome, cheaper.

It will also examine the rationale for the Wyangala and Mole River projects, the Macquarie River re-regulating storage project and the Western Weirs project.

The Committee will hand down its report in the first half of 2021, with an interim report expected in 2020. The inquiry's terms of reference have not yet been approved by its members, but they are expected to be signed off by Wednesday.

Support the local news that keeps you informed - subscribe today.