That is the warning from UNSW Sydney's Professor Richard Kingsford who is an expert in conservation biology, wetland and river management with the School of Biological, Earth and Environmental Sciences.

Professor Kingsford gave the warning while addressing the NSW Government's inquiry into the rationale for, and impacts of, new dams and other water infrastructure in NSW.

The inquiry held its final hearing on December 4, 2020.

"It would essentially take out the large floods and allow that storage of water over a number of years to deliver the 21 gigalitres estimated to be delivered," Prof Kingsford told the inquiry.

"The issue is: Where does that water go? That water stays in that river but essentially would not go much further than probably the Jemalong irrigation area, so the damage done to the river system is primarily downstream of that.

"Water will still flow down the river, but it flows only to a point where it is taken out; downstream of that is where you see the real impact. The major wetlands like the Booligal system, the Cumbung system and even Lake Cowal and the Lachlan swamps will be the major places of damage in terms of what happens.

"The other thing to say about this is that our rivers have always naturally been boom and bust systems, so our plants and animals are used to that.

"When we put a large dam in, we regulate that, so we essentially are able to control the flows."

Prof Kingsford explained to the inquiry that the better flows are not necessarily beneficial to the environment and native fish species.

"What has happened in a lot of our rivers is that we do provide more low flows, which are important in terms of delivering that water for irrigation, towns and stock and domestic, but it also means that we are providing better habitats for things like carp," he said.

"They love living in constant water and when the rivers do dry up, the native fish species are much better at being able to deal with that sort of natural system and carp really benefit from the constant flows in the rivers."

Prof Kingsford went on to tell the inquiry: "We need to manage our rivers much better so that we have a enough water in the dams for essential supply".

"Most of the water in those dams is for irrigation; it is not for critical human needs."

In reply to a question from the inquiry on what difference a debit system for water would have made to the Macquarie and Lachlan valleys and what difference it would have made to irrigators in years preceding drought, Prof Kingsford admitted it would probably have meant irrigators received less water.

"They probably would have got less water because you would have held some water back to ensure that you are able to carry that river over into the very dry periods," he said.

"If you had a wet period, they would have got the same. It is an issue really about managing for climate change."

He went on to tell the inquiry that there are other options for water security that need further investigation.

"The government has got some really good options in some of its regional strategies," he said.

"There are things like off-river storages, and in other parts of Australia, particularly Western Australia, they use underground storages in terms of aquifers for holding water.

"Obviously one of the things we are not very good at in many places in Australia is that when we think about water we go, 'Supply option: we have got to find more water'.

"You do not create more water, you cannot make it rain more, so you are essentially taking water from somebody else.

"But we can put a lot more effort and energy into the demand part of the equation-in other words, making more efficient use of water.

"In the Lachlan there is a great opportunity for a win-win where we have got essentially a very inefficient irrigation system that has not been a beneficiary of the Federal Government's efficiency program.

"In fact, some of that water that the Federal Government could invest would increase the potential irrigation output and free up some of that water that could actually be used for town water supply.

"When we think about some of the arguments about drought proofing and water security, they are already there in what we have got if we got a lot more imaginative and did not just go for the old supply option of: Where can we build another dam"," Prof Kingsford said.

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