Three threats have been reported at Sanral's operations centre in the past week.
A bomb threat was reported at the SA National Roads Agency Limited's (Sanral) operations centre in Samrand, near Midrand on Sunday.
Electronic Toll Collection (ETC) CEO Jamie Surkont said a sweep of the building was in progress and if necessary, they would move to the back up sites with the relevant services.
"The system, as per the previous threats, remains operational, with limited services, such as the call centre affected. All systems are successfully being monitored from the back up facilities," Surkont said in a statement.
This was the third threat to be reported at the centre this week.
The premises were evacuated twice this week after a suspicious powder, initially thought to be anthrax was found.
ETC later reported that the substance was harmless.
Transport Minister Dipuo Peters on Sunday condemned the threats.
She called on law enforcement authorities to act swiftly in their investigation to identifying those responsible for the threats.
"There are elements in society hell-bent on undermining the peace and
security of their fellow citizens. These backward and uncivilised individuals have elected to ignore all platforms made available to them by our Constitution to raise whatever concerns they may have," Peters said in a statement.
"We call on law enforcement authorities to do all within their powers to bring the perpetrators of these acts to book."
Those affected were receiving the necessary attention and help in dealing with the traumatic experiences of the past week, said Peters. –Sapa