Scientists are urging major dam projects in NSW and Queensland be abandoned, warning of significant environmental and agricultural consequences.
ANU Professor Jamie Pittock, a member of the Wentworth group of concerned scientists, said he Wyangala dam proposal in NSW is expected to cost more than $2 billion dollars, with alternatives not properly explored.
"There are many alternatives to increase water security for all ... it can't be full to supply water and empty to catch a flood," Professor Pittock told a conference on Wednesday.
He called on the NSW government to abandon Wyangala expansion plans, as well as Dungowan Dam near Tamworth, and Queensland to abandon the Hells Gates project.
"The proposal to raise Wyangala Dam is an exemplar of why these dam-building projects are crazy," he told AAP.
"It was proposed without a business case."
Prof Pittock, who has studied water policy for two decades, said the proposals are "unjustified and ill-conceived water management interventions", and called on the federal government to abandon them.
The NSW government has said money for dams is contingent on a co-funding agreement with Canberra.
"If that money was spent on the alternatives, could the alternatives deliver the same or better benefits with less impact?" Prof Pittock asked on Wednesday.
"There are better ways at reducing flood risk and better ways for securing water for agriculture."
Farmers, traditional owners, scientists and politicians have told a two-day Listening To The Lachlan conference about the impact NSW's dam proposals will have on the river.
Internationally renowned expert on Australia's water resources and water birds Richard Kingsford also wants the projects scrapped.
"Native fish, waterbirds, frogs, river red gums will decline in numbers across the river catchment with flow-on effects across the Murray-Darling Basin," he told AAP.
The UNSW professor said Wyangala dam's expansion will devastate wetlands downstream on the Lachlan River.
"Essentially this dam is going to take those flood plain flows and put it in the dam."
"This river will change...it will dry from the bottom up," he told the conference.
Lake Cowal Foundation's Mal Carnegie, who is working with the Lachlan Environmental Water Advisory Group, told AAP the Wyangala proposal had not adequately considered downstream water users or the environment.
"You have to consider the bigger picture because it's such a big impact on the whole system ... It's potentially disastrous," he said.
On Tuesday the head of the Lachlan Valley water group - which has advocated for the dam wall to be increased for flood mitigation and water storage - also spoke at the conference.
Mary Ewing told the gathering crop losses from the 2016 floods were estimated at $500 million and any business case needs to be thorough.
Chair of the Lachlan Valley water group, Forbes farmer Tom Green, told AAP early studies show raising the wall would be beneficial.
"We believe the project has shown it will be highly effective for both flood management and water security," he said.
Mayors along the river, from the Forbes, Cowra and Lachlan shires, remain committed to raising the wall.
Forbes mayor Phyllis Miller told AAP the dam needs to go ahead.
"To those people that believe raising the wall and inundating some land up in the dam catchment is an environmental problem, let them come see what a flood does," she said.
NSW Water Minister Kevin Anderson told AAP: "I look forward to continuing discussions with the new federal government and receiving a commitment to funding these crucial water infrastructure projects.
"Water security is critical to the survival and the future of regional communities."
An environmental impact statement is expected after August for the Wyangala project, while work has begun on stage one of the Dungowan Dam pipeline.
A Queensland government official said a business case for the Hells Gates Dam in the state's north is due to be sent to the federal government in June.
Federal Water Minister Tanya Plibersek was unavailable for comment.
