It argues a "fit-for-purpose" new weir would allow native fish to migrate and water for the environment to pass through, in keeping with strict laws and a water sharing plan.

Environmentalists have expressed concern that the new weir could reduce small and medium flows to Warren and the Macquarie Marshes.

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But WaterNSW executive manager assets Ronan Magaharan insists the new weir will "benefit all water users".

"Some of the information circulating about this proposed replacement weir overlooks the fact that there is already a significant barrier in the river at Gin Gin," he said.

"In contrast the proposed replacement weir is a modern, gated structure, designed to benefit all customers, including the environment.

"The gates can be lifted so tributary flows and floods can pass through, or lowered to temporarily store water for delayed delivery and even maintain natural river flows several metres lower than the existing weir if needed."

The manager said the new weir would be able to hold and release small volumes of water to the "far reaches of the river system", reducing the evaporation which occurred when it was "making its way down the river from the other side of Wellington".

Mr Magaharan said the new weir would make it possible for fish to migrate "upstream and downstream in that stretch of river for the first time in over a century".

"This upgraded infrastructure helps WaterNSW regulate the river for the long-term benefit of all users and must comply with the strict environmental regulations that oversee its operating conditions, and the legislated NSW government water sharing plans which ensure equity and fairness among competing interests," he said.

WaterNSW calls the existing flood-damaged and structurally-deteriorating Gin Gin weir "a barrier to native fish movement and flow variability" as well as an "outdated and ungated concrete wall incapable of of being operated to enhance river management".

The information sessions will be on August 18 from 6pm to 7pm; August 19 from 12pm to 1pm; September 2 from 6pm to 7pm, and September 3 from 12pm to 1pm.

To participate, join facebook.com/groups/MRRRS.

Visit www.waternsw.com.au/mrrrs for more information on the project.