"Three young boys illegally gained entry into the facility through an opening in the palisade fence," spokesman Mandlakazi Mpahlwa-Sigcawu said.
A 12-year-old boy drowned in a reservoir in Vosloorus, east of Johannesburg, the Ekurhuleni municipality said on Tuesday.
"Three young boys illegally gained entry into the facility through an opening in the palisade fence," spokesman Mandlakazi Mpahlwa-Sigcawu said.
"One of the boys went inside the meter-deep floating roof of the reservoir which was filled with rain water and he was unable to come out."
The incident happened on Saturday afternoon.
Mpahlwa-Sigcawu said police were investigating.
The reservoir had been broken into repeatedly over the past few months and over 15 steel manhole covers were stolen. Concrete covers were used to replace the stolen ones.