South African utility Eskom said on Tuesday it would implement rolling blackouts from 4pm until 10pm, and again the following day, after breakdowns in some generation units left the electricity system severely constrained.

In a statement, Eskom said six generators had been returned to service at its Medupi, Tutuka, Kendal, Majuba and Grootvlei power stations the previous day, but the breakdown of four units overnight and on Tuesday morning, as well as a delay in the expected return to service of one, had left the grid under pressure.

This made it necessary to apply 'stage 2' load-shedding entailing the suppression of 2 000 megawatts of demand to avoid tripping the grid.

"Any further deterioration in the generation performance may ... necessitate the escalation of load shedding at short notice," Eskom added.

The entity, which supplies about 95 percent of South Africa's electricity, most of it coal-fired, has been forced to apply load shedding on and off in recent years due to breakdowns largely attributed to years of inadequate maintenance of its infrastructure.

It did so again for a couple of days last week, citing an increase in breakdowns.

"As the aged generation infrastructure is unreliable and volatile, this constrained power system is expected to persist for the rest of the week, particularly as the cold front hits," Eskom said on Tuesday, referring to the weather service's predictions of what is likely to be the final cold spell in parts of South Africa as the winter season draws to a close.

The utility said unplanned breakdowns currently stood at more than 11 900 megawatts of capacity, adding to the 4 350 MW currently out on planned maintenance.

Eskom is one of several state-owned companies beset with financial problems largely due to mismanagement.

Several former senior executives have left the utility after being implicated in the so-called "state capture" corruption scandal in which the wealthy Gupta family allegedly conspired with politicians in former president Jacob Zuma's administration to influence decision-making in order to advance their own interests.