"To see photos of it is sad, but when you actually see it in the flesh, and you see hundreds of Murray cod piled up on top of each other, just rotting away, the smell; that's when it really hits," he said.

"It really breaks your heart."

Behind the deaths is toxic levels of blackwater in the Murray-Darling Basin, a naturally occurring event where organic material, such as leaf litter, is swept into a waterway by flooding and its rapid decay sucks dissolved oxygen from the water.

An increase in temperature can cause dissolved oxygen to drop and at very low levels it can cause 'hypoxic' water which makes it difficult for fish and other aquatic animals to survive.

Blackwater also makes it harder for water to be treated for human consumption.

Western Murray Land Improvement Group (WMLIG) executive officer Roger Knight told ACM that as a result of extensive flooding along the basin throughout Victoria and NSW in 2022, dissolved oxygen levels had dropped well below the normal range of 6-8mg/L.

"[In 2022] we've generally hovered in the Murray River above 0mg/L but below 2mg/L so we've gotten to some really low levels; below 0.5mg/L for about a week in this event," he said.

"For a couple of weeks we've started seeing shrimp come out of the water and congregate near the edge and die, crays are coming out of the water and there have been some fish deaths."

Australian National University Fenner School of Environment and Society Professor Jamie Pittock told ACM the blackwater issues were not unlike what occurred along the river after major flooding events in 2010, 2011 and 2016.

Although this year's event has not been as severe as in the past. In 2010, dissolved oxygen levels in the river near Barham in NSW were undetectable for six months.

"Ironically, one of the issues here is that there's too infrequent flooding, so before development of the rivers the rivers would naturally have flowed on to the floodplain ... every year or every second year and the leaf litter wouldn't get as much of an opportunity to build up," Professor Pittock said.

In 2013, the federal, Victorian, NSW and South Australian governments agreed to implement large scale solutions under the Murray Darling Basin Authority (MDBA) constraints management strategy to 'investigate how water can get to where it's needed'.

"[The strategy] is about the governments working with land holders on the floodplain to purchase flood easements across private property and working ... to relocate or raise or strengthen infrastructure that might go underwater like low lying roads and bridges," Professor Pittock said.

"That's so the environmental water that's being held in big dams ... can be released out in a pulse that fills up the river channel and spills that over onto the floodplain.

"That will all diminish the amount of leaf litter on the floodplain and that would prevent or dramatically reduce these big blackwater events."

Dams such as Burrinjuck, Hume and Eildon are designed to hold back smaller and medium sized floods that previously would have flushed out floodplains, and are used instead to run that water out to supply irrigated agriculture in drier months.

WMLIG's Mr Knight said there was support among river communities for this strategy that could deliver highly oxygenated water to rivers and creeks across the basin.

"A lot of fishermen and others have been really vocal about the need to continue that and to make sure that there's information out there so people can make rational informed decisions about the fact that, you know, it's not new water that's coming from nowhere; if it wasn't coming down as irrigation systems it would be going in the river somewhere," he said.

"It's been overwhelming that people want more water in the landscape, especially people downstream in the rivers at the bottom end of the system."

MDBA's deadline to implement and deliver on the constraints management strategy promises is 2024.

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The authority's executive director Andrew Reynolds said it was monitoring increased water quality risks as the weather warmed and water flowed from floodplains that had not had water on them "for many years".

"Governments and water authorities continue to work together to monitor hypoxic blackwater conditions and blue-green algae which can lead to fish deaths and water quality issues in the Murray-Darling Basin," Mr Reynolds said.

"While there have been fish deaths associated with this year's floods and resulting low-oxygen hypoxic blackwater, the reported fish deaths are not unusual for floods of this magnitude."

The federal minister for water Tanya Plibersek will hold a ministerial meeting on February 13 where it's anticipated she will address what the government intends to do to implement its election promises including the constraints management strategy.

In the meantime, communities across the basin have taken action into their own hands, fundraising to buy aerators for rivers, creeks and lakes 'so [fish] can breathe a little easier until the most deoxygenated flows pass through'.

A spokesperson for NSW Department of Primary Industries said it understood fish death events could be distressing to the local community.

"DPI Fisheries staff respond to fish deaths and poor water quality, working with NSW and Commonwealth agencies to implement available management options such as managing water releases to dilute flows where possible, as well as the potential use of irrigation system 'escapes' to deliver oxygenated water and create small areas of refugia," the spokesperson said.

"Strategic rescues of priority native fish species are undertaken where and when appropriate."

Similarly, a spokesperson for the Victorian Department of Environment, Land, Water and Planning said works to boost the health of native fish were under way.

"Water quality monitoring and native fish rescue efforts are under way in several locations across northern Victoria and in partnership with NSW for the Murray River," the spokesperson said.

"Almost 2,000 native fish and crayfish have been relocated to other waterways with higher oxygen levels or to dedicated hatcheries and other facilities as a temporary home until conditions improve and they can be returned."

SA's Department of Primary Industries and Regions (PIRSA) confirmed no blackwater events had yet been detected or reported in the state but, if one occurred, "PIRSA has an emergency response plan in place to respond to large-scale fish kill events".

The widespread flooding that prompted the blackwater event across the basin can be attributed to above average rainfall in 2022 as Australia's eastern states sloshed their way through a third consecutive La Nina weather event in three years.

The Bureau of Meteorology has forecast La Nina conditions will end in Feburary 2023 and move to an 'ENSO-neutral'.

University of NSW researcher Professor Richard Kingsford has been surveying wetlands for almost four decades and said within months the river will show some positive flow-on affects from La Nina to aquatic-plants and invertebrates, which may help blackwater issues in the basin.

"What happens is essentially when the plants start to grow and the invertebrates are there, they actually do a fantastic job of cleaning that water up and actually making it a lot cleaner when it comes out the other end," he said.

"If you see water coming out of a big wetland like the lower Murrumbidgee wetlands or the Macquarie Marshes, by that time it's been essentially filtered by the wetland plants," he said.

And the longer-term positives of La Nina for wetland ecosystems will be seen for years to come.

"The water will also go in and replenish groundwater supplies. So the effects of this can last for [up to] 10 years so it is really positive and it's the sort of thing that we've been changing by building dams and taking water out of rivers," Professor Kingsford said.

It's a future for the rivers, creeks and wetlands along the basin that seems especially far away given the warm weather ahead.

"We don't know what's coming in front of us. We've got some hot weather now and in the next few weeks," Western Murray Land Improvement Group's Roger Knight said.

"We've seen dissolved oxygen drop when we've had a couple of warm days. We've been really lucky with the cold weather we've had from a fish perspective for dissolved oxygen.

"We're not out of the woods yet."

If people do notice something unusual along the river, they are encouraged to contact the relevant hotline in their state or territory.