An "elevated mortality" event at a salmon farm in northern Tasmania is being blamed on higher water temperatures.
Aquaculture company Petuna says its farm at Rowella experienced a temperature spike in an unusually short time frame, followed by a prolonged period of long, hot days.
It is not known how many fish have died, but Environment Protection Authority (EPA) director Wes Ford says the number is in the thousands.
He said higher temperatures started about a week ago and increased mortality "progressed over the course of the last seven days".
"At the moment they're having to take fish out on a daily basis and send them to disposal. It's not a small loss," he told ABC radio on Tuesday.
Petuna says temperatures are now decreasing and disposal of fish will be managed in line with EPA regulations.
"For now, our immediate concern is to safeguard those fish that have not been affected by removing those that have perished," CEO Ruben Alvarez said in a statement.
An EPA spokeswoman said the regulator was determining the extent of the fish deaths.
According to Petuna's website, the Tamar River produces around 2800 tonnes of salmon per year and employs 25 people.
"Strong river flows and tidal movements at Rowella lend themselves to a continual supply of well oxygenated water," it reads.
"The site also benefits from its isolation from other marine farms, with particularly good biosecurity and environmental outcomes."
Mr Ford said the EPA had received reports from other companies of minor increases in salmon mortality rates, something he said was pretty typical for this time of year.
Tasmania's salmon industry has received criticism in recent months from environmentalists who say it is not as clean and green as it claims.
The federal and state governments in September announced an agreement to support a framework to deliver aquaculture in deeper waters off Tasmania.
