A 33-year-old Mpumalanga police constable is in custody after he allegedly hijacked a couple while on duty, provincial police said.Police spokesperson Brigadier Leonard Hlathi said on Wednesday that the constable, who was on duty and dove a police vehicle, signalled at a motorist to stop."The policeman then went to the vehicle and ordered the driver, together with his wife, to alight from it," Hlathi said.The constable - who was based at the Balfour police station - then allegedly hijacked the vehicle at gunpoint and left his state vehicle behind.Read: 5 Western Cape cops arrested for corruption, drug dealingHe was arrested on Tuesday night when he handed himself over to Mpumalanga police."Police recovered the victim's vehicle as well as the state firearm," Hlathi continued.Mpumalanga police commissioner Lieutenant General Mondli Zuma has since expressed concern about "illicit behaviour by police officials"."Police are expected to be the custodians of the law and not the ones breaking it. The police will not hesitate to arrest their own if found to be on the wrong side of the law," he said.Read more: Ekurhuleni metro cops nabbed for briberyThe constable faces charges of hijacking, the use of a state vehicle without consent and the theft of a firearm.He is expected to appear in the Balfour Magistrate's Court on Wednesday.