The water company was prosecuted by the Environment Agency (EA) after sewage spilled from a pumping station near Great Yarmouth into the Ormesby Little Broad in February last year.
The site is connected to Ormesby Broad, where drinking water is abstracted by Essex and Suffolk Water. However, the drinking water was not affected by the spill.
The EA said toxic levels of ammonia and high biological oxygen demand had been spilled and there was also potential for harm to the public.
The pumping station has a history of overflowing with five reported incidents between September 2006 and March 2011.
“As a result of previous overflows, Anglian Water was fully aware of the risk that an overflow from this pumping station could cause a polluting discharge to the Ormesby Little Broad,” said Clare Clorfield prosecuting for the EA. “However, they failed to take appropriate measures to prevent a recurrence.”
The company was fined £20,000 and £2,896 in legal costs.