South Africa's Oscar Pistorius will not be released under correctional supervision this Friday because the decision to do so was made without the right legal basis, the country's justice minister has said in a statement.

Pistorius was expected to be released after serving 10 months of a five-year sentence for killing his model and law graduate girlfriend Reeva Steenkamp on Valentine's Day 2013.

Justice Minister Michael Masutha said in a statement that the parole board had decided to release Pistorius on parole even before the athlete had served a sixth of his sentence, as required by law.

"It is therefore clear that there is no legal basis upon which such a decision was made ... one sixth of a five years sentence is 10 months and at the time the decision was made Mr Pistorius had served only over six months of his sentence," Mr Masutha said.

The minister said he received a petition from the Progressive Women's Movement of South Africa, opposing Pistorius' release on parole, saying it flouted the rules.

A family member said he was shocked by the minister's decision, adding that the family had planned a "low key welcome" for Pistorius on Friday.

"We are shocked and disappointed that Oscar won't be home this Friday," the family member, who declined to be named, told Reuters.

Pistorius has admitted killing Steenkamp, 29, by firing four shots through the locked door of a toilet cubicle, saying he believed an intruder was hiding behind it.

Judge Thokozile Masipa said during sentencing the state had failed to convince her of Pistorius' intent to kill when he fired.

Prosecutors want the verdict of culpable homicide, equivalent to manslaughter, raised to murder because they argue Pistorius must have known when he fired that the person behind the door could be killed.