Hundreds of thousands of fish have been found dead in NSW's far west, three weeks after an unprecedented fish kill struck the area, with the state government again blaming the "environmental catastrophe" on weather.
Regional Water Minister Niall Blair on Monday afternoon confirmed the deaths at Menindee weir and along parts of the Darling River affecting bony herring, golden perch and carp.
"The conditions out there are showing a lot of fish that are near the surface of the water, gasping for air," he told reporters in Sydney.
He said it is an "environmental catastrophe" and conditions could potentially deteriorate further.
"No scientist, no locals, no one has been able to point to anything else that could prevent something like this other than fresh water coming into the system and we just don't have that," Mr Blair said.
Fisheries officers from the NSW Department of Primary Industries were on high alert across the weekend, relocating some struggling Murray cod.
Reports were also received of fish deaths in Lake Keepit and Thredbo.
In a statement, the department said the latest fish kill is the result of "critically low levels of dissolved oxygen" likely linked to the mixing of weir pool water following the significant drop in temperature over the last few days.
A Bureau of Meteorology spokesman told AAP temperatures at Menindee peaked at 48.8C on Friday, falling to 23.3C on Sunday, but said "that's not unusual for this time of year".
Menindee resident Rob Gregory said there were up to 300 dead bony bream, and some native species.
"There are lots of yabbies crawling up the bank ... they must be suffocating," he told AAP.
Up to a million fish died at Menindee earlier in January, some of them decades old, while thousands were found dead days later almost 900km away along the Macintyre River.
In an interim report released on January 24, the NSW government said the first Menindee "ecological disaster" was caused by low rainfall and large temperature variants leading to an algal bloom that sucked oxygen from the water.
Mr Blair said water aerators have been installed and dismissed suggestions that cotton farmers and big industry were taking more than their share, while also welcoming information about any such behaviour.
The government has also pointed the finger at Canberra and the Murray-Darling Basin Authority for authorising water releases from the Menindee basin when water levels were higher.
"There will be plenty of people that will dissect the events that led up to this," Mr Blair said, citing two federal investigations currently under way.
"I'm sure that the control and water releases will all be part of those debates."
NSW Labor leader Michael Daley wants the government to "abandon" its current approach and announce a comprehensive water plan including the reintroduction of the cease-to-pump rule for the affected system.
Mr Blair will travel to Menindee on Monday night to see the latest damage first-hand.
The council has hired a clean-up operator which will also record information about the event.
Australian Associated Press