Oscar Pistorius was convicted of murder on Thursday by South Africa's Supreme Court of Appeal, which threw out his earlier conviction of the lesser crime of culpable homicide for shooting dead his girlfriend.

"Guilty of murder, with the accused having criminal intent," judge Eric Leach told the court in a dramatic legal reversal.

"The matter is referred back to the trial court to consider an appropriate sentence."

The 29-year-old star Paralympic sprinter shot dead Reeva Steenkamp, a model and law graduate, in the early hours of Valentine's Day in 2013.

He said during his trial last year that he mistook her for an intruder when he opened fire at the locked door of his bedroom toilet.

He was released from prison on parole in October after serving one year of his five-year sentence for culpable homicide — the equivalent of manslaughter.

"He did not know whether that person constituted any threat," Leach said in a damning rejection of the testimony from Pistorius and the original trial judge's ruling.

"It is inconceivable that a rational person thought he was entitled to fire at this person with a heavy-duty firearm," said Leach, who described Pistorius's testimony as "vacillating and untruthful".

"He must have foreseen that the person behind the door might be injured.

"He ought to have been convicted not of culpable homicide on that count but of culpable murder."