It is estimated that hundreds of thousands of native fish died in the north New South Wales after the rains, he washed ashes and silt from forest fires on the Macleay River.
Parts of the Macleay River – favored by recreational fishermen – have been transformed into what the locals described as a “dripping cake mix” that stank of rotting vegetation and dead fish.
A freshwater ecologist told Guardian Australia that the impact of killing fish could be felt in the coming decades, with long-lived species like Australian sea bass being hit hard.
The NSW Department of Primary Industries has received reports of "hundreds of thousands" of fish killed in the river since December 2019.
Locals say the rain in the past 10 days has seen more ash and mud from the dry, scorched landscape running into the river.
The disaster on the Macleay River is one of eight fish deaths reported to the department this year, with the cause mostly related to the lack of rain.
Larry Newberry, a recreational fisherman from Frederickton, near Kempsey, said he drove 100 km to George's Creek to survey the river last weekend.
"I would say that, from what I saw, I would not be surprised to end all fish in at least 100 kilometers of the river," he said.
“The stench was overwhelming – it stank so much it made you shiver. It is the dead fish, the rotten vegetation and the ashes of the fires and perhaps the fire retardant. It's like brown slime.
"I have been fishing in the river for 50 years and I have seen dead fish before, but nothing of this magnitude. This will happen in all rivers on the east coast hit by forest fires."
Newberry criticized the commercial fishing operations that captured Australian sea bass and also the main industry department for what he called a lack of response.
The species seen dead and reported to Guardian Australia were Australian sea bass, eels, mullet, mullet, herring eyes, herring, gudgeons and catfish.
Upstream of Kempsey, in the town of Bellbrook, residents use pumps and hoses borrowed from firefighters to try to oxygenate the water.
Newberry said he admired the efforts, but feared it was "like pissing 40 miles an hour no less."
James Pritchard, founder of the Bellbrook Social Fishing Club, said the rain on Thursday raised the river's level, but "brought tons and tons of debris and dirt with it".
He said: "Now there is more ash in the river than I have ever seen before. The top of the river is covered in ash. The water looks like a mixture of cake dripping. It is terrible.
"The river is gone for generations – I won't see it come back in my life. To say that I'm gutted is not the word."
Aboriginal elders relied on a healthy river to feed and teach culture, he said, and that would devastate those efforts.
"This is so wrong. DPI knew it was going to happen and they didn't put anything in place," he said.
Professor Lee Baumgartner, a freshwater ecologist at Charles Sturt University, said the fish would be choked.
The addition of ash and nutrients to the water promotes bacteria, which in turn remove oxygen from the water. And if the water gets muddy, the fish won't be able to pass enough water over their gills to extract oxygen.
While efforts to oxygenate the water may seem futile, he said that even a saved mature female could continue to spawn and lay hundreds and thousands of eggs.
He said there was a precedent for understanding the long-term impacts of an event like this: large forest fires in 1939 caused ashes to flow into the Lachlan River and "the fish never recovered".
He said: "I think we will see more of these events while it is raining. These things can have decades of impact. It can be really terrible.
The Department "fish killsThe page also features reports of hundreds of dead fish in Lake Tilba, on the south coast of NSW, where there was soot on the banks, alongside golden, flat, mullet, eel and blue swimming crabs.
The drought probably also killed thousands of fish in the Hastings River, near Port Macquarie, the page reports.
A NSW DPI statement said its fisheries department "continues to investigate a fish death event in the Macleay River".
The statement said: “The suspected cause of the incident is poor water quality, leading to low dissolved oxygen. Rain events are adding ash from extensive forest fires across the region to local watersheds, in addition to other organic matter and sediment. This can cause rapid drops in oxygen levels in the water.
“The fishing team conducted numerous field assessments and the main affected species were Australian sea bass, freshwater mullet and catfish. The number of impacted fish is estimated at hundreds of thousands. "
Community members were encouraged to report fish deaths or observations to the Fishers Watch hotline on 1800 043 536.