Twenty-eight female guards were unfairly dismissed by a security company because the client‚ Metrora.
The South African Transport and Allied Workers Union has labelled bus company SA Roadlink a "union basher".
The union has also called on the Department of Labour to publicly arrange an urgent inspection of all SA Roadlink buses and its employees' working conditions.
Satawu's sector coordinator Assaria Mataboge said yesterday that they had been trying since 2005 to recruit the majority of the workers at the bus company.
Mataboge said: "Every time we went to make a presentation to management we were met with hostility. Employees were keen but could not sign up for fear of losing their jobs."
He said management had always threatened to fire staff who dared to join a union.
"Satawu is aware that one of the fundamental causes could be that drivers do not get enough rest as they operate trips as single drivers when the norm in the industry is for two drivers to be used," said Mataboge.
Herman Steyn, SA Roadlink risk manager, said the allegations were baseless.
He, however, admitted that the employees did not belong to any union.
"We have never been approached by any union at any of our branches.
"We have never threatened any employee who wanted to join a union," Steyn said.
Department of Transport spokesman Collen Msibi said the chairman of the operating licence board was investigating whether SA Roadlink had complied with the requirements of its licence.
"Minister Jeff Radebe will make a decision regarding the bus company as soon as the investigation was finished," he said.
Msibi said the company had so far been banned from operating in KwaZulu-Natal only.