The incident took place on Thursday afternoon when an excavator deployed to clean up waste accidentally hit the pipeline, Transnet said.
"Due to a fire on the adjacent landfill site, the Transnet Pipelines team could not access the pipeline to establish the full extent of the damage and as a safety precaution the pipeline was isolated and gas flow through the pipeline was stopped," the utility said in a statement.
Repairs to the pipeline began on Friday morning after the fire was brought under control on Thursday night, it added.
Transnet did not say when repairs could be completed but said it was working to restore supplies "as soon as possible".
The gas pipeline runs from Secunda in Mpumalanga to the port city of Durban, transporting an average of 450 million cubic metres of gas per year.
(Reporting by Nelson Banya; Editing by David Goodman)