© Allan GratzerOne of dozens of dead stingrays found on the shore of Lake Conjola, north of Ulladulla.
About 30 dead stingrays washed up on the shore at Lake Conjola, north of Ulladulla.
The NSW Department of Primary Industries, which has taken samples, said the likely cause of death was extremely cold water, between 8 and 9 degrees Celsius.
In a statement, the department said the "most probable" cause of death was the water cooling rapidly over a short period of time.
It said there were no visible signs of any pollutant, and no other species were affected.
The president of the Conjola District Lake Care Association, Robin Kerves, said she was not convinced the water's temperature was to blame.
"I should imagine that our water has been a lot colder than what it is now," she said.
"We've never had mass deaths like this before."
She said opening the lake to the ocean may be key to saving the rays in future.
"They appear to be down that end, trying to get out to the ocean, and if the lake was dredged, we'd have a proper flow," she said.
The Shoalhaven City Council was conducting a separate investigation.
Council's environmental health officer Andrew Gibbes said while the kill was unusual, there was nothing to suggest there was anything wrong with the water's quality.
"I haven't any reason to suspect any different, but obviously, if the situation changes or indeed the water quality testing we're having done shows anything, naturally, we'd have to look at that situation again," he said.
"At this stage, everything's indicating just a natural phenomenon and there's no danger to public health or environmental health."
About 60 stingrays were believed to be still alive in the lake.
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