Student constable Sipho Mbatha had no reason to feel threatened by the Soweto teenager he allegedly shot dead, the High Court in Johannesburg heard on Monday.

"There's nothing that could have threatened him. Thato had surrendered. He wasn't making any moves that could have made Mbatha feel threatened," Constable Tshepo Mokhomola testified.

Mokhomola was one of the six officers who was present when Thato Mokoka, 16, was shot dead, allegedly by Mbatha, outside his grandmother's house in Bramfischerville on February 14 this year.

Police had been at the house searching for Mokoka, who was alleged to be part of the BWA gang (Boys With Advices), and to own an illegal firearm. The firearm was never found.

Mbatha has pleaded not guilty to the murder charge against him. He said his R5 rifle went off accidentally as he tried to search Mokoka, who was lying on his stomach.

Mokhomola told the court another officer had already searched Mokoka before he was taken out of the shack, at the back of the house, which he and several other youths had been in.

"I would have expected Mbatha to have seen what was happening (Thato being searched) because he was supposed to be keeping watch on us and taking cover as we went in," said Mokhomola.

The court was told how another officer said there was no firearm and had suggested that they all leave, but Mbatha stayed behind.

"I decided to go back and check for the firearm, and Mbatha was still holding the firearm over Mokoka. When I was inside, searching still, I heard the shots," said Mokhomola.

"After the shooting, he threw the firearm down and held his head. He said (in Zulu) 'what have I done?' and held his hands on his head."

The gun initially had 20 rounds, and 13 were left after the shooting.

Mokhomola said visibility was good as police had told the occupants of the shack to switch on lights.

Last week however the teenager's aunt, Mpumi Mokoka, testified that there was no electricity in the shack. She said the only light came from a bulb in the house.

The trial continues.