The Supreme Court of Appeal in South Africa has changed Oscar Pistorius's culpable homicide conviction for killing his girlfriend Reeva Steenkamp to murder.

The six-time Paralympic champion was handed the less serious conviction last year after he testified that he mistook Steenkamp for an intruder when fired four gunshots through a toilet door in the early hours of Valentine's Day in 2013.

The 29-year-old, who was sentenced to five years, spent less than 12 months behind bars in Pretoria before being transferred to house arrest at his uncle's luxury home, but his long-term future remained uncertain as prosecutors lodged an appeal to the court against the original conviction.

Today, Justice Eric Leach upgraded the conviction to murder after detailing the case and the five-person panel's reasons of why they cannot accept Pistorius's version of events.

Leach, who claimed that the original judgement was "fundamentally flawed", told the court in Bloemfontein that Pistorius "never offered an acceptable explanation" as to why he fired gunshots through the door, and that the death of the person in the cubicle was an obvious result of his shooting.

"I have no doubt the accused must have foreseen and therefore did foresee that whoever was behind that door might die," Leah told the court. "The identity of his victim is irrelevant to his guilt."

Pistorius will be re-sentenced at the High Court in Pretoria in January and faces a minimum of 15 years behind bars.

Reeva's mother June Steenkamp was in court to hear the appeal outcome.