The National Union of Mineworkers expressed concern on Friday after six workers died at an Eskom construction site in Ladysmith, KwaZulu-Natal.
"As the NUM we have always emphasised the need of the department of labour to reinforce its health and safety inspectors in the construction and energy sector," said spokesman Erick Gcilitshana.
"We are calling for an immediate investigation into what went wrong. We believe that such tragic incidents can be prevented."
The accident happened at Eskom's Ingula pumped storage scheme near Ladysmith on Thursday.
The men were doing welding and grouting on a platform in one of four tunnels when the platform came loose and rolled down the tunnel.
All 3387 workers on site were evacuated.
Eskom said the site would be closed until it was declared safe.
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