A 200-metre section of Christies Beach in Adelaide's south has been closed to swimming after a break-in overnight caused 300,000 litres of sludge to overflow into Christie Creek.

Key points: Section of Christies Beach closed following sludge spill into nearby creek

Section of Christies Beach closed following sludge spill into nearby creek The overflow was caused by a break-in to a wastewater treatment facility

The overflow was caused by a break-in to a wastewater treatment facility Swimmers have been warned to stay out of the water until tests results are back

Initial investigations indicate that the sludge overflow from the Christies Beach Wastewater Treatment Plant had been contained to the creek, but as a precaution swimmers have been advised to stay out of the sea water at a section of Christies Beach.

South Australian police are investigating the incident and water samples have been collected from the beach, with results expected tomorrow.

SA Water's production and treatment acting senior manager Dr Daniel Hoefels said the break-in happened overnight on Sunday and some equipment was manually operated on site, which had caused the sludge overflow.

"As a precautionary measure, SA Health has advised swimmers to stay out of the sea water 200 metres immediately adjacent to Christie Creek," Dr Hoefels said.

A warning sign which was erected after a sewage spill at Christies Beach in Adelaide. ( ABC News: Simon Goodes )

"We're working closely with South Australian police on their investigation into the illegal break-in, we're also working closely with the EPA to ensure that the creek is properly cleaned."

Dr Hoefels said the sludge that was flowing into the creek was a by-product of the wastewater facility and approximately 300,000 litres had made its way into the creek.

He said temporary signage had been erected to warn swimmers in the area.

"Sludge is a by-product of the wastewater treatment process. Put simply, it's some of the solid material that is removed from wastewater," he said.

"At the moment it appears that the majority of the sludge is retained within the creek … we've undertaken a range of water quality tests and they are due back tomorrow.

"We've got temporary bunting in place and at this time it appears that the sludge has been contained to the creek."

Symptoms of diarrhoea is 'worst case'

Principal water quality adviser from SA Health, Dr David Cunliffe, said while the health risks to swimmers was low, the worst case could mean people end up with a gastrointestinal illness.

"In this case the risk is low … most of the sludge has been contained to the creek and little or no sludge has entered the water but as a precaution we're advising swimmers to avoid a 200-metre stretch of water, 100 metres either side of Christie Creek," he said.

"In the unlikely event that some sludge has entered the water and if somebody then swam in that water, the worst case is that they could develop a gastrointestinal illness involving diarrhoea and possibly vomiting.

"As soon as the incident was identified SA Water personnel in the area, signs were erected very quickly so people were advised not to swim."

He said if people who had swam in the area noticed symptoms of illness, to contact their medical practitioner.

Dr Hoefels said water samples had been taken from the area today and results were expected back on Tuesday.

He said the 200-metre stretch of the beach would remain closed until test results could confirm the sea water was unaffected.

The Christies Beach Wastewater Treatment Plant has since returned to normal operation.