Untreated wastewater has been released into rivers south of Adelaide and into Gulf St Vincent, prompting a health warning.
SA Health says a fault at the Normanville Wastewater Treatment Plant released about one megalitre of untreated water into the Normanville Creek and the Bungala River.
People have been urged to avoid contact with the river water.
An area has also been cordoned off 100 metres each side of the river outlet on Normanville Beach.
SA Health water quality adviser David Cunliffe said people who come into contact with untreated wastewater could experience gastrointestinal problems.
SA Water senior manager Lisa Hannant said an investigation was underway into the incident.
"Our top priority will be understanding what happened, to make sure it doesn't happen again," Ms Hannant said.
"The plant has returned to normal operation and additional monitoring processes are in place while our investigation is underway."
Ms Hannant said SA Water would also work with the Environment Protection Authority to assess localised clean-up options and any necessary actions to monitor and rehabilitate the affected watercourses.
Water samples have been collected from several downstream locations from the treatment plant to determine the extent of the discharge, with results expected within the next 24 hours.