A police ballistics expert offered key testimony in the trial of South African athlete Oscar Pistorius on Wednesday as the court sought to establish whether he shot his girlfriend intentionally.

Reeva Steenkamp was first hit in the hip and was in a defensive position in the toilet stall of Pistorius's flat when the South African Paralympic star's last shot hit her head in the early hours of Valentine's Day 2013, Chris Mangena testified.

"The best probable explanation is the deceased was initially upright behind the closed door, she sustained a penetrating wound in the right side of the hip," Mangena said.

The 29-year-old model and law graduate then fell onto a magazine rack where another bullet hit her right elbow, he said.

The last bullet hit her head, clipping her raised left hand, Mangena said.

"The deceased was seated in a defensive position," he said. "The arm was lifted up, and was in front of her chest."

"You've both hands forward, with the left and right hand crossing the head?" asked prosecutor Gerrie Nel, mimicking the position with his own arms.

"That is correct," his witness answered.

The sequence of shots speaks to whether Steenkamp could have screamed during the shooting.

Witnesses have testified to a woman's terrified screams, but the defence say she was hit first in the head and therefore could not have screamed.

Mangena also testified that the athlete could have been as close as 60 centimetres (24 inches) to the toilet door, or as far away as 220 centimetres, when he fired.

The distance from the door could shed light on whether Pistorius fired on the cubicle in a panic or approached it in a calculated plan to hit his target.

The ballistics report will be key in the state's case of premeditated murder against Pistorius.

The sprinter says Steenkamp's death was a terrible accident, and that he shot her through the locked toilet door after he mistook her for an intruder when she went to the toilet.

Pistorius, 27, sat with his head in his hands, while Steenkamp's mother and relatives in the public gallery lowered their heads when graphic images of the blood-splattered bathroom were shown.