Up to 3,700 megalitres of environmental water will be released from Copeton Dam over a 28-day period.
According to a NSW Office of Environment and Heritage (OEH) statement, the water will flow into the Mehi River, downstream of Moree in the Gwydir catchment.
OEH Director Conservation Programs Derek Rutherford said native fish and river flows will benefit from the release, which began yesterday.
"This flow will be followed by a coordinated release of Commonwealth and NSW water from the Dumaresq and Macintyre Rivers, and the Gwydir River, to replenish waterhole refuges for native fish in the Barwon River at least as far downstream as the Macquarie River junction," Mr Rutherford said in a statement.
"Detailed planning for this subsequent flow is currently being finalised with the Commonwealth Environmental Water Office," he said. The Mehi River is in the Barwon catchment in north-west NSW within the Murray-Darling Basin.
It rises near Pallamallawa as the southern offshoot of the Gwydir River, called the Big Leather Watercourse, and flows generally west, before meeting the Barwon River near Collarenebri.
The planned flow is endorsed by the Gwydir Environmental Water Advisory Group consisting of community members, landholders, water users, Indigenous representatives and independent scientists.
OEH is working in partnership with the Commonwealth Environmental Water Office, DPI Fisheries, Department of Industry (Water) and WaterNSW to deliver the flows.
Water releases trigger a surge in the number of insects and micro-organisms within a wetland. Frogs emerge to feed and become food for waterbirds, fish, turtles and other reptiles.
Native fish respond to the conditions and begin to breed. Fish are an essential part of a healthy river.
Supporting native fish supports the recreational fishing and tourism industries.
For more information on water for the environment, visit: www.environment.nsw.gov.au/topics/water/water-for-the-environment/about-water-for-the-environment/what-is-it