Samwu, whose members have been on strike since Monday, on Thursday rejected the employer’s latest wage offer.
Unions representing municipal workers were inching closer to a wage deal, an official said ahead of a meeting with the employer on Thursday.
Municipal worker unions are set to decide on Wednesday if a new wage offer would be accepted as a national strike enters its third day.
The strike by municipal workers is costing the country in the region of R15-million a day, an economist said on Tuesday.
South African municipal workers are to strike over wages from Monday, unions said, in a move that could hit public services.