Near the New South Wales-Victorian border, fish rescuer Braeden Lampard walks into a flood-soaked wheat field. There he finds decades-old Murray cod struggling to breathe.
"I don't think anyone has ever caught a Murray cod in a wheat crop. I didn't think it was possible," he told ABC's 7.30.
"It was sad to see and really sad to witness."
Native fish are desperately seeking refuge because parts of the Murray-Darling Basin have turned toxic due to hypoxic blackwater. Everything that lives in the affected water is literally suffocating.
"Just today, we covered an area of approximately two kilometres. I counted [more than] 100 dead Murray cod," Mr Lampard said.
"If you have a couple hundred fish dying every 500 metres or kilometre, you have that possibility of tens of thousands, maybe even hundreds of thousands of native fish dying."
The blackwater is a consequence of the unprecedented flooding that's occurred in the south-east of Australia.
Leaves, grass and other debris are washed off riverbanks and flood plains into waterways. Bacteria break down the organic material, but Kyalite farmer Daniel Andrews said the process sucked up vital oxygen in the water and fish die.
"It's a tea colour, really black, as if you had a really strong tea. It's full of debris … and there's black, thick black slime on the top," he said.
"The water is a putrid stench at the moment that you nearly have to wear a mask. I'm standing here in front of it [and it] just makes you feel sick inside.
"It's an environmental disaster on a monumental scale."
'It's just a big kick in the guts'
Some of the dead fish were up to 40 years old.
It's a huge blow for river communities because fish stocks depleted by drought were bouncing back. More than 1 million fish were estimated to have died across the Murray-Darling Basin during the big dry.
"We've had low rivers and low water allocations and then we finally get river systems running and then we get a fish kill like this. It's just a big kick in the guts," Mr Andrews said.
"I love fishing. I love native species and it just breaks my heart."
E. coli, chemicals such as PFAS and pesticides have been detected in floodwater, at levels not considered harmful. However, Victoria's Environment Protection Authority is recommending people not swim nor eat from contaminated waters.
Josh Standen is among the landowners who source water directly from the river.
"We shower in this water and, looking at what's next to me here, I wouldn't put my worst enemy under a shower, under this stuff," he told ABC's 7.30.
"People are going to have to start filtering their water just to have a shower."
The outbreak of toxic water is expected to worsen.
"In a week's time, I think it's meant to warm up a bit. All these lagoons and stuff are going to heat up and these [dead] fish numbers are going to go up tenfold. There'll be a lot more fish kills," Mr Standen said.
The local water utility, Lower Murray Water, said making the river water safe for drinking was taking longer than normal and it has warned restrictions could be introduced over the warmer months to curb usage.
Jarod Lyon from Victoria's Department of Environment, Land, Water and Planning said oxygen levels in the water would likely drop further.
"The further you move into summer, the faster that microbial bacterial action occurs and that speeds up the rate at which the oxygen was taken out of the water," he said.
"I think, as a fish ecologist and someone who's worked in the river restoration game for a long time, I know myself and my colleagues are really, really worried about this situation, really upset."
Rescue mission
With time running out, not-for-profit conservation group OzFish is working alongside state government agencies to save as many native fish and crustaceans as possible. Mr Lampard is leading the rescue mission.
"Our volunteers have been able to save 46 native fish. So these fish include Murray cod, trout cod, silver perch and golden perch," he said.
"We've also been able to save 650 freshwater crayfish."
The rescued fish are being taken to an aquaculture facility in Mildura and will be returned to the river once it's safe.
As the floodwaters move downstream, it's feared South Australia will also face a mass fish kill. Volunteers are preparing for a rescue operation there too.
"So the blackwater event will approximately last for another two to three months, so we're looking into early 2023," he said.
"We aim to continue these rescue missions for as long as it takes to save as many fish as we possibly can.
"The more fish we save, the less (sic) fish die."
The severity of the blackwater event has ignited debate about river management under the Murray-Darling Basin Plan, with some farmers claiming water set aside for the environment has worsened the situation.
Dr Lyon said it was too early to lay any blame beyond the unprecedented floodwaters.
"We think the size, the scale of these floods, is the major issue here," he said.
"There will be a review into this. After the event, people will get out and do some research. They'll try and work out what's going on."
The Commonwealth Environmental Water Holder, Simon Banks, said there was no evidence to support claims environmental flows had made hypoxic blackwater events worse. On the contrary, Dr Banks said, they helped reduce the impacts.
"Environmental flows provided in winter and spring flush forests and woodlands while temperatures are cooler, reducing the build-up of organic matter," Mr Banks said in a statement.
"In some cases, environmental flows can be used to provide small, localised refuge for fish and other aquatic species to escape hypoxic conditions. This is currently happening in the Edward-Wakool region."
Mr Andrews said he was worried it would take years for the river system to return to health.
"I've never seen anything like it and I don't know how long [it will be] before the river systems can recover after this," he said.
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