A power outage in SA's main coal export terminal Richards Bay has forced state-owned logistics monopoly firm Transnet to delay offloading of trains for 10 hours, the company said on Friday.

"The outage meant that the Richards Bay Coal Terminal was unable to offload trains for approximately 10 hours, resulting in a 50% wagon capacity for Friday's production plan," the freight logistics group said in a statement.

Africa's most industrialised economy faces regular power outages as it struggles to generate enough electricity to keep the lights on, affecting mining and other large businesses relying heavily on the country's only power utility Eskom.

These companies, especially coal miners, are further completely dependent on Transnet's rail freight infrastructure for evacuation, transportation and off-loading.

Therefore any disruption to power supply or transport infrastructure hurts exports and revenues.

Transnet said the outage occurred from Thursday and into the early hours of Friday at the east coast port of Richards Bay, the main conduit for high-quality coal exports mainly to Asian markets, including India, China and Pakistan.

Transnet said a "catch-up" plan was underway and a revised schedule has been communicated to customers.

On Thursday, Exxaro Resources, South Africa's biggest coal miner, said its exports would remain subdued in 2021 as rail logistics problems, including derailments and cable theft, stymie its export ambitions.