The houses were those of Transnet employees at the Ngqura Container Terminal, general manager of terminals in the province Siyabulela Mhlaluka said in a statement.
He said this appeared to be part of “a co-ordinated campaign of terror and intimidation”.
The National Union of Metalworkers of SA has been on strike at the terminal over transport allowances, working hours for particular tasks, and the use of labour brokers.
The union could not immediately be reached for comment.
Mhlaluka said 18 non-striking workers had been subjected to such attacks, and the company would step up security measures.
“In the early days of the strike, non-striking colleagues’ houses were attacked, but the bout of attacks subsided after interventions by Transnet and the police. The new wave of violence has since resurfaced on Wednesday.”
Transnet has offered a reward of R100,000 for information leading to the arrest and successful prosecution of the perpetrators.
As of Monday, all Transnet terminal employees were directly employed by the company, following a decision to do away with labour brokers in its core operations.