A SA National Roads Agency (Sanral) employee has been arrested for acts of terrorism relating to the white powder scare at its offices, the Hawks said on Thursday.

"Yesterday afternoon [Wednesday], the Organised Crime Unit of the Hawks in Gauteng arrested a 28-year-old employee of the South African National Road Agency (Sanral) on charges relating to acts of terrorism," Captain Paul Ramaloko said in a statement.

"The man was arrested at Sanral's Central Operations Centre in Centurion while on duty. During his arrest a cellphone, suspected of having been an instrument used to commit the offence, was seized."

Sanral employees were evacuated from their Midrand offices in Gauteng on three different occasions recently after two white powder scares - which turned out to be harmless - and a bomb threat.

"The motive for the suspect's alleged offence is unknown, investigations are continuing," said Ramaloko.

The employee is expected to appear in the Pretoria Magistrate's Court on Friday on charges relating to acts of terrorism.