On Wednesday, November 6, fisherman Greg Hardman captured a video which shows a number of dead fish floating in the Combadello Weir, about 25km south-west of Moree.

Fellow fisherman Greg Brown believes many of the fish killed were natives including yellowbelly, silver perch, Murray cod and bony bream.

"It's terrible," he said.

"We think it was the black water, and a lack of oxygen that killed the fish. The rivers get so low that stagnant pools form. The good water comes down and picks up the stagnant pools and it turns into black water which sucks the oxygen out."

Department of Primary Industries (DPI) Fisheries staff have since cleaned up the dead fish, with samples taken to try to determine the cause of the mass fish kill.

According to Northern Tablelands MP Adam Marshall, staff estimate about 200 fish were killed.

Mr Brown said he hopes they can try to prevent this from happening again.

"I hope the scientists and fisheries can find a cure so this doesn't happen again," he said.

"Maybe the water could have come earlier, or a bit faster, I don't know.

"We've had it before and I suppose we'll get it again but my God, I hope not.

"It upsets fisherman. We like to have a little bit of water for the fish to swim in."

DPI Fisheries have been contacted to see if there has been a cause of death determined however no response has been provided.

Days after the Commonwealth Government ordered an environmental flow out of Copeton Dam into the Mehi River, a mass fish kill has been discovered near Moree.

Although the cause of this death is yet to be determined, Minister for Agriculture Adam Marshall said a sudden change in the water system could precipitate fish deaths.

"There are multiple reasons why we see fish deaths on mass in river systems, typically it's because of a lack of oxygen in the water.

"It can be because of a sudden change in temperature or nutrients entering the system, like we saw at Lake Inverell earlier this year where we had a huge storm and it pushed a lot of organic matter into the Lake causing the fish to suffocate, but that's not been determined," he said.

Department of Primary Industries (DPI) Fisheries discovered the fish deaths on Tuesday and have had staff cleaning up.

Minister Marshall said staff estimated about 200 fish dead, many of those Carp but some natives, including Murray Cod, as well.

"Staff are taking some of those fish away to determine the cause of death. This comes as no surprise to me because this is exactly what all the experts and scientists have been warning me as Fisheries Minister," he said.

The deadly mix of less water and warm weather means inevitably there will be more fish deaths this year round than last summer.

Minister Marshall has been working with local anglers to rescue and relocate native fish in a bid to preserve numbers and genetics.

"We're doing everything we can to rescue thousands and thousands of fish and breed them so we can replenish the fish lost and replace them with millions more," he said.

The water recently released out of Copeton Dam for environmental flow was an order by the Federal Government to keep water in the river system and the Gwydir Wetlands.

"Because it's water owned by the Commonwealth they can call on that water at any time and it has to be released.

"There is not a lot we can do in terms of not allowing the flow but there is no doubt those flows can have impacts, not to say that is what caused these fish to die; we just don't know that yet," he said.

DPI Fisheries have been contacted to see if there has been a cause of death determined however no response was provided at the time of print.