Anne Davies watches fish dying on the Darling River in western NSW after a mass kill just a week ago

“It’s happening again,” says Menindee local Graeme McCrabb, as we stand on the banks of the Darling River, downstream from the fish kill on 7 January that resulted in the deaths of hundreds of thousands of fish, including 70-year-old Murray cod.

“We stood here watching that guy die about half an hour ago,” he says gesturing to the white belly of an 80cm Murray cod floating in the middle of the stream.

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Close in to the bank, another big cod is swimming slowly, just below the surface, gasping as it tries to get oxygen from a weak current flowing there. He’s too big to get into the area with faster current, but the water is probably too hot anyway because it’s flowing over a sand bank. Its 40C here this morning.

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Further out, two more cod are near a sand bank barely moving as they search for a current.

“That one’s dying,” says McCrabb, as it turns on its side and we briefly glimpse its white belly.

McCrabb and his friend Paul Grose, have been visiting these water holes daily, to assess the health of the fish.

Today they are close to tears.

It's heartbreaking, seeing these big fish die Paul Grose

“I am gutted,” McCrabb says. “I’ve had a couple of moments this morning.”

“It’s heartbreaking,” says Grose. “Seeing these big fish die.”

I ask: “Is this normal, to see them swimming like this?”

“Shit no,” says McCrabb. “They usually live on the bottom.”

“It’s worse in the morning because the oxygen levels are lower,” he says as we watch this appalling scene of majestic fish gasping for life. “The algae will start to photosynthesise as the day goes on, but then, the water’s getting really warm in places and that’s no good either.”

Facebook Twitter Pinterest Dead carp in the Darling River above the weir at Menindee. Residents fear conditions are lining up for another mass fish kill a week after hundreds of thousands died. Photograph: Mike Bowers/The Guardian

Then there is the fear of what will happen if the temperature drops sharply. A change is forecast for Friday night, which could see a repeat of the conditions that occurred before the massive fish kill of last week in the weir pool. Hundreds of thousands of fish were left strewn along the banks of the Darling: small bony bream, large Murray cod, golden perch , silver perch and even carp.

In the mean time the temperatures is expected to hit 45C again on Thursday, and there is no rain in sight.

Quick guide Heatwaves and climate change in Australia Show Hide According to the Bureau of Meteorology and the CSIRO, Australia has warmed by 1C since 1910, and temperatures will increase in the future. So how will climate change affect future heatwaves in Australia? The CSIRO and BoM have compiled different models for predicting the outcome of climate change in Australia to produce a guide to how different regions will likely be affected. They found that every part of Australia will continue to experience increases in average temperature, and will have a higher frequency of hot days.

The duration of hot spells will increase in every region. In many areas in the northern half of Australia, the average number of days above 35C could increase by two to three times.

Late in the century, towns such as Darwin, Alice Springs and Broome may experience days with temperatures above 35C for about a third of the year. These higher temperatures will also result in higher evaporation, which will continue to make drought conditions worse.

The New South Wales Department of Primary Industries has said the massive kill was due to cooler weather killing the algal blooms, deoxygenating the water and killing already stressed fish.

“The big fear is this section of the river has the strongest cod populations,” McCrabb says. “We’ve been watching it like hawks. We were going to try and do a dogdy save – move them to another pool, but we don’t think the cod would survive.”

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The DPI staff are in Menindee now installing aerators at two places. But they only provide refuges and cannot make any significant change to the oxygen levels in the river.

The chief executive of the Murray Darling Basin Authority has cast doubt on whether the solar powered devices, which are used in acquaculture, will make any real difference.

The place where we watch the fish dying is downstream of the aerators and the fish cannot reach them.

“This is the biggest environmental catastrophe in the river’s history and no one from the federal government has been here,” McCrabb says.