Five workers of the Sol Plaatje Municipality were killed at Kimberley's Homevale waste water treatment plant, a city official said on Tuesday.
“We confirm the death of five of our workers…at around 12.45pm today (Tuesday),” said city spokesman Sello Matsie.
He said one of the workers was cleaning the pump house, about six metres underground, when the accident happened.
Apparently sludge came out of one of the monitors in the room and filled the room to a height of about 1.5 metres.
It was suspected the worker was overcome by methane gas.
Matsie said it seemed the other workers tried to help in an attempt to rescue the first worker and they were also overcome by the gas.
He said the accident was now subject to various investigations by different state departments, the police and the municipality.
The recovery of the bodies was nearly completed by 6.30pm on Tuesday.
Matsie said questions about safety issues would be handled in the investigation. - Sapa