Oscar Pistorius could serve at least 15 years in prison after a South African appeals court found him guilty of murdering his girlfriend, Reeva Steenkamp.

Last year, Pistorius was convicted of ‘culpable homicide’, or manslaughter, over the shooting death of Ms Steenkamp on Valentine’s Day in 2013.

He was sentenced to five years in prison after shooting her four times through a locked toilet door.

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The former Olympian, nicknamed “Blade Runner”, spent one year in prison before he was released on house arrest, but will now have to return to the court room to be re-sentenced.

On appeal, prosecutors argued that Pistorius should be convicted of murder.

Judge Eric Leach told the court Pistorius was “guilty of murder, with the accused having criminal intent”.

“This is a human tragedy of Shakespearean proportions,” he said.

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

June Steenkamp, left, the mother of Reeva Steenkamp, is comforted as she listens to the appeals court. Photo: AAP

He found the lower court did not correctly apply the rule of dolus eventualis – whether Pistorius knew that a death would be a likely result of his actions.

Pistorius denies deliberately killing Ms Steenkamp and said during his trial last year that he mistook her for an intruder before he fired the shots.

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

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

State prosecutors who lodged the appeal say Pistorius intended to kill Steenkamp and that she fled to a toilet during a row.

The father of the slain model says he felt a sense of “relief” after South Africa’s appeal court found that Pistorius was guilty of murdering Reeva.

“It’s a big relief. I feel it’s a fair decision that the judge gave,” Barry Steenkamp said in a brief interview on local television station ANN7, before breaking down in tears.

Judge Thokozile Masipa, who originally found Pistorius guilty of South Africa’s equivalent of manslaughter, will re-sentence the Paralympic champion at a later date.

In South Africa, a murder conviction usually carries a minimum sentence of 15 years, although Pistorius’ lawyers are likely to argue that his physical disability and mental stress should be considered as mitigating circumstances.

The Paralympic gold medallist was released on parole in October after serving a fifth of his five-year prison term for the “culpable homicide” of Steenkamp.

-with AAP