South African Olympic and Paralympic athlete Oscar Pistorius has taken to the witness stand in his own defence at his murder trial in Pretoria.

Mr Pistorius said the shooting of his girlfriend Reeva Steenkamp had left him sleepless, terrified and plagued by nightmares.

He also apologised to Ms Steenkamp's mother in the public gallery.

He said he fired four shots through a toilet door in the early hours of Valentine's Day last year in the belief that he was defending her from an intruder.

"I was simply trying to protect Reeva," he told the court.

Mr Pistorius is accused of killing Ms Steenkamp deliberately by firing four rounds from a pistol through a locked toilet door after an argument.

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

Earlier, Mr Pistorius wept and retched into a bucket in court during graphic forensic testimony about the death of Ms Steenkamp.

As the first defence witness, forensic pathologist Jannie Botha, described the multiple bullet wounds sustained by Ms Steenkamp, the 27-year-old sat in the dock with his head buried in his hands.

He vomited occasionally into a plastic bucket.

This also happened during previous sessions last month when the court heard details of Ms Steenkamp's autopsy and reviewed pictures of the blood-stained toilet and bathroom.

If found guilty of murder Mr Pistorius faces at least 25 years in prison.

During his testimony, Mr Botha reviewed Ms Steenkamp's wounds and the sequence in which the bullets hit her.

This is a crucial detail since several witnesses have testified to hearing a woman's screams during a volley of shots in the early hours of 14 February 2013.

Mr Botha agreed with state pathologist Gert Saayman that the first round hit her in the hip, causing her to collapse, while the final shot hit her in the head, killing her almost instantly.

The trial has gripped South Africa and millions of athletics fans around the world.

The sprinter's lower legs were amputated as a baby but he went on to achieve global fame as the "fastest man on no legs".

He won gold medals at the Beijing and London Paralympics, and reached the semi-finals of the 400 metres in the London Olympics against able-bodied athletes.