TasWater says a Cadbury chocolate factory, owned by Mondelez International, discharged a "big spike of sugary water" into the facility, killing the Cameron Bay wastewater plant bacteria that eat sewage.
"This has meant that only partially treated water is now entering the Derwent (River) hence the do not swim notices," community relations manager Matt Balfe told reporters on Friday.
"We're really disappointed that this has happened at a time in the year when families are gathering around the foreshore of the Derwent, particularly the week that children are leaving school."
Partially treated water is entering the Derwent River, according to TasWater. (Rob Blakers/AAP PHOTOS)
Mondelez said it was working with TasWater to investigate and resolve the matter."We have recently undertaken a $3 million upgrade of Mondelez International's wastewater treatment plant which prepares milk solids and other manufacturing by-products for safe disposal," it said in a statement.
"The health and safety of our community and protecting the environment are our utmost priorities."
Mr Balfe said new bacteria would be transplanted into the sewage treatment plant but it could take up to a week before the facility, which continues to pump waste into the river, was discharging safe levels of effluent.
"These are bugs that are needing to come back to life and start to generate their own process of eating that sewerage," he said.
Mr Balfe said the the bacteria's health started declining three weeks ago and they started dying off 48 hours ago before hitting a "critical mass" late on Thursday that resulted in the treatment process failing.
About 4,500,000 litres of waste is pumped into the river per day.
TasWater has since ordered Mondelez to stop sending industrial waste from its Claremont factory to the sewage plant.
"Without the bugs there to treat that effluent we can't keep receiving that, and it will take the ... bugs being back online with this reseeding process for us to be able to accept their waste again," Mr Balfe said.
TasWater and the Environment Protection Authority are working with Mondelez to improve its waste processing.
The Tasmanian Health Department has issued a do not swim warning for the River Derwent between Austins Ferry and Old Beach in the north, and Sandy Bay across to Howrah in the south.
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"Sewage in water used for recreational activities like swimming poses a risk to health from viruses and bacteria," Director of Public Health Mark Veitch said.
"These risks can include gastroenteritis (diarrhoea or vomiting) and infections of the skin, ears or eyes."
The Tasmanian Greens said Mondelez 's actions were "shocking" and called for an investigation
"Mondelez International has been spewing waste into the northern Hobart sewage treatment plant for weeks now, remaining non-compliant with regulations despite TasWater's work with them," Member for Clark Helen Burnet said."How is it that this breach has taken so long to be made public by TasWater and Mondelez?"
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