More than 1,000 miners are awaiting evacuation from the Beatrix mine in Free State province, some 122km (76 miles) north of the city of Bloemfontein.
The workers are not believed to be in immediate danger, but mine unions have expressed concern that they could suffer from dehydration and lack access to medication.
Livhuwani Mammburu, a spokesman for the National Union of Mineworkers, said: “There is no electricity there, so they are using a generator to power the cage to take workers to the surface.
“The problem is that they are rescuing one mineworker at a time. It is very slow. It is worrying. Some of our members have chronic conditions and they don’t have medication down there.”
Food and water is being delivered to those trapped in the mine, which is run by the Sibanye-Stillwater company – the largest individual producer of gold in the country.
James Wellsted, a spokesman for the firm, said the thunderstorm on Thursday had resulted in a power outage, which had stopped the shaft lift from working.
“We sent rescue teams down, accounted for all the people and obviously are trying to get them out of there as soon as possible,” he said.
“All employees appear to be fine. We are supplying food and water.”
Sixty-five people were rescued during the day, but as many as 1,200 are still thought to be trapped.
Engineers are said to be struggling to get emergency generators working in order to re-start the lift, which serves one of four operating shafts in the mine.