The document sent to customers and dated on Monday said the force majeure would be implemented with immediate effect. It would impact container terminals in Durban, Ngqura, Port Elizabeth and Cape Town due to "an act of cyberattack, security intrusion and sabotage" which has disrupted normal processes and "continues to persist".
Transnet's official website was still down and showing an error message on Tuesday.
Transnet said in the notice that it was putting mitigation measures in place, including the manual loading and discharge of containers, to ensure continued operations at the container terminals although at a slower pace.
Some employees not involved in running operations were forced to take leave while the company resolved issues related to the cyberattack, two sources with direct knowledge said.
Transnet did not immediately respond to a request for comment.
The company, which operates major South African ports, including Durban and Cape Town as well as a railway network that transports minerals and other commodities for export, confirmed its IT applications were experiencing disruptions last week.
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