Oscar Pistorius was angry after a policeman reprimanded him for leaving his gun on the seat when he was stopped for speeding, High Court in Pretoria heard on Friday.

His ex-girlfriend Samantha Taylor told the court that Pistorius later fired a shot out of the car, "perhaps out of spite," she said.

"After we left, he was angry. Before he fired the shot he was irritated and afterwards he laughed," said Taylor.

Pistorius, Taylor and his friend Darren Fresco had been driving from the Vaal when the incident occurred.

Taylor testified that Pistorius initially said he wanted to shoot at a traffic light, and then suddenly fired a shot through the open sunroof of the car.

As the 20-year-old spoke of the incident, she lifted her right hand in the air and demonstrated a shooting motion.

Taylor told the court she and Pistorius had broken up because he had cheated on her with Reeva Steenkamp.

Pistorius is on trial after shooting Steenkamp through a locked bathroom door at his Pretoria home last year.

He says he mistook her for an intruder.