South African Olympic and Paralympic sprinter Oscar Pistorius broke down in tears in the witness stand, forcing the adjournment of his murder trial.

It happened as he recalled the moment he discovered he had shot dead his girlfriend Reeva Steenkamp.

"I sat over Reeva," the athlete testified before sobbing loudly, putting his head in his hands and letting out a barely intelligible cry "she wasn't breathing".

The trial has adjourned until tomorrow to allow the athlete to regain composure.

Earlier, Mr Pistorius retched on the witness stand as he described the moments leading up to the shooting of Ms Steenkamp, on Valentine's Day last year.

He has pleaded not guilty to murder, saying he mistook the 29-year-old model and law graduate for an intruder hiding in the toilet at his Pretoria home.

During a morning of dramatic court testimony, Mr Pistorius recounted how he heard a window sliding open in his bathroom in the middle of the night, convincing him an intruder was breaking in and that he needed to arm himself.

"That's the moment that everything changed," he said.

"I thought that there was a burglar that was gaining entry to my home."

When post-mortem examination pictures of Ms Steenkamp's bloodstained body briefly flashed up on the courtroom television monitors, the 27-year-old athlete doubled over in the witness stand, retching into his cupped hands.

Mr Pistorius fired four rounds through the door, three of which hit Ms Steenkamp, killing her almost instantly.

He is accused of killing Ms Steenkamp deliberately by firing from a pistol through the locked toilet door after an argument. 

He says he fired in the belief an intruder was behind the door.

The testimony follows an emotional first day that saw him issue a tearful apology to the victim's family.

Mr Pistorius sat before a packed courtroom as he described how his relationship with Ms Steenkamp developed, and with faltering voice, how the couple began to think of the future.

The paralympian described how he met the model at a track-day car event. "The first six days we knew each other we called each other everyday," he said.

"We both had things that kept us back. We both came out of difficult relationships before," he said.

"I was very keen on Reeva."

"I think if anything I was more into her, than she was at times with me."