This is a feature of lowland river systems and occurs during flooding when organic material - such as sticks, leaves, bark and grass - is broken down in the flood water or washed off the floodplain into the river.

Current watchpoints in the region that are being monitored include Murray River communities from Tocumwal to the South Australian border, the Murrumbidgee River downstream of Narrandera, including the Lowbidgee floodplain from Maude to Balranald and the Edward Wakool system in the region of Deniliquin and Moulamein.

Andrew Reynolds, MDBA executive director of river management, said the first flush of water after a long drought could make blackwater events more likely, and all governments and water authorities were working together to manage the unfolding situation.

"After three hot and dry years, floodplains across our river systems have a high load of leaf litter - much of which hasn't moved since 2016," he said.

"The current weather pattern, as well as the high chance of La Nina conditions in spring, increases the risk of floods.

"When rains wash organic matter into waterways, it can lead to hypoxic blackwater events, which suck the oxygen out of the water.

"Fish and other aquatic animals have difficulty surviving."

The last major hypoxic blackwater events in the southern inland valleys occurred in 2016.

Director of the Water Planning Implementation for the Department of Planning, Industry and Environment, Allan Raine, said agencies are continuing to keep a close eye on conditions throughout the region.

"In many cases mitigation measures to resolve blackwater events are limited but water agencies will work together to determine if and when any management options are available," Mr Raine said.

Anyone who spots any signs is urged to notify the department of potential blackwater events by emailing waterqualitydata@industry.nsw.gov.au.

To report a fish kill, members of the community can contact the NSW Fisheries Hotline on 1800 043 536.