Johannesburg - Workers at Medupi power station in Lephalale were still locked out of the premises, the National Union of Metalworkers of SA said on Monday.
"The employer [Eskom] wants workers to sign final warning letters and we are against that. Numsa is also not prepared to sign a peace agreement which clearly prohibits the right to strike properly," said deputy general secretary Karl Cloete.
The union was opposed to final written warning letters and advised the employer to instead issue general warnings.
Construction work at Medupi came to a halt on January 16 after a dispute about the payment of year-end bonuses.
Forty-six workers were arrested on February 15 following violent protests.
Protesting workers torched an Eskom vehicle and damaged a police vehicle during the protest.
Eskom temporarily closed the power station when contract workers went on strike. Construction at the coal-fired power plant, which is set to be commissioned in 2015, was also interrupted when workers downed tools in September.
Efforts to obtain comment from Eskom on Monday afternoon were unsuccessful.