Armed with a bucket and a pair of tongs, the nine-year-old Stawell resident attempted to save as many animals as she could.

"She was upset to see the dead turtles, yabbies and fish, and the ones still alive in the rocks," Mahli's mother Molly said.

"She spent two days catching as many of the animals as she could and moving them to the nearby creek in buckets."

Mahli said she was devastated when noticed all the dead animals.

"It was a bit gross but I wanted to clean it up as a respect to the animals and it would be better for little children not to see the dead animals," she said.

"The turtles dead in the drain probably upset me the most.

"Turtles go through a lot with the plastic that gets into our water ways.

"If we knew about it was going to happen we could do something to help."

In this instance, hundreds of animals were beyond saving.

A video taken by Stawell resident 'Fish' Seehusen on December 28 showed the extent of dead wildlife, with dozens of turtles piled up at the exit point of a dry pipeline.

The Wimmera Catchment Authority CEO Dave Brennan said they were investigating the "operational issues" that caused the "unfortunate" situation.

Mr Brennan said the authority had requested an environmental flow to be released from Lake Lonsdale into the Mount William Creek before Christmas.

GWM Water reported the lake was just 14 per cent full on December 23 - significantly lower than it was at the same time in 2019 at 23 per cent full.

"It was very much an unintentional operational issue that has happened," Mr Brennan said.

"We had requested a quick environmental release to freshen up the Mount William Creek below Lake Lonsdale towards Dadswell Bridge.

"It appears that when the valve was shut to the lake, a number of fish and turtles were essentially stranded or beached as the water receded. That is our assumption at this stage.

"GWM actually operate ... the release of water, so we'll be working closely with them to understand what caused the problem to prevent anything like this happening again."

GWM Water operational manager Mark Williams said GWM followed plans and management strategies specified by the Wimmera Catchment Authority.

"They make the order, and we execute the order," he said.

"We have to investigate it further in regards to what has transpired, but to the best of my knowledge, I am satisfied that our release program has been consistent with the orders of the catchment authority."

Mr Williams said rain at the weekend had helped increase water levels at Lake Lonsdale, however water tended to dissipate quickly from the shallow lake during the summer period.

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Lake Lonsdale remained 13 per cent full on January 6, according to GWM's latest data.

Mr Brennan said managing water levels was a complicated process, and the authority acted in the best interest of all waterways, not just lakes.

"One of the challenges about our role is we need to take a holistic approach for the entire catchment area, not just one section," he said.

"Each year, as there are inflows going into Lake Lonsdale, a percentage of those flows need to be passed through.

"All of the water isn't just harvested in the lake and nothing goes below into the Mount William Creek.

"What we were doing is basically letting the passing flow that has been accumulated over the last few months to be released down to the river.

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"Some of the water has to be turned over. It's a system, not to be viewed in isolation."

Mr Brennan said there were "trade offs and tough decisions" the Wimmera Catchment Authority had to make to maintain healthy waterways.

"I don't want to downplay the incident, but the amount of species and animals that have died compared to the total amount in the lake, we're talking about a very small percentage," he said.

"It was not an absolute disaster for the lake or river system, but it is far from ideal.

"Ideally we'd like to have no deaths. But whenever we're moving water around, there are those possibilities."

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Mr Brennan also thanked people like Mahli who helped salvage some of the animals.

Mahli however was disheartened by the number of animals that could not be saved.

"She saved many buckets of yabbies and redfin fish," her mother said.

"Unfortunately, the turtles were all dead."

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