Eskom has been buying coal from 19 South African coal mines which do not have water licences, for at least two years, it was reported on Monday.
Under the National Water Act it is illegal for a mine to start up and operate without a water licence, Beeld reported.
According to the newspaper, Eskom has blamed the department of water affairs, and said the water licences should be issued.
"Between November 2012 and March 31, 2013, none of the providers (at the department of water affairs) who lodged applications for a water licence received that licence," Eskom spokeswoman Hilary Joffe told Beeld.
Eskom had in the past reportedly repeatedly stressed that it bought coal only from legitimate mines which met all environmental requirements.
Beeld reported that, in reply to a parliamentary question about Eskom, Public Enterprises Minister Malusi Gigaba said it was prerequisite for Eskom that the mines from which it purchased coal complied "fully" with environmental legislation.
The department had since acknowledged that some mines were without licences and were illegally mining coal.
However, it argued that the water affairs department was struggling with an administrative backlog which prevented it from timeously issuing licences.
Water affairs spokesman Sputnik Ratau told Beeld nobody should mine without a water licence. "It's illegal."
Eskom reportedly refused to give Beeld the names of the offending mines.