Oscar Pistorius is with members of his family at his uncle's mansion after being released from prison and moved to house arrest a day earlier than expected, a spokeswoman said.

"Oscar is here, and Oscar is at home with the family," spokeswoman Anneliese Burgess said, addressing dozens of reporters outside Arnold Pistorius's luxurious home in an upmarket suburb of South Africa's capital Pretoria.

"The family is happy that Oscar is at home," Ms Burgess said.

Pistorius, the double-amputee runner who fatally shot girlfriend Reeva Steenkamp in 2013, was moved from a central Pretoria jail on Monday night when he had been expected to be released on Tuesday.

He has served a year of his five-year sentence for manslaughter for killing Ms Steenkamp. Under South African law, he is eligible to serve the remainder under correctional supervision, a form of house arrest.

The decision to release Pistorius, 28, a day earlier was only communicated to his family at short-notice, Ms Burgess said. Confirming Pistorius's release, South Africa's Department of Corrections said that the decision over when and how an offender is released is made by the prison.

"The handling of the actual placement is an operational matter of the local management, and how they handle it is their prerogative that is carried out in the best interest of all parties concerned, the victims, the offender and the Department of Correction Services," Manelisi Wolela, a spokesman for the department, said.

The murder trial of Pistorius generated intense international interest, and the surprising decision to release Pistorius a day early, and at night, appeared to have avoided the logistical challenges and spectacle associated with a large gathering of TV crews and other journalists hoping to catch a glimpse of Pistorius on the way out of prison.

Ms Burgess was surrounded by reporters and camera crews when reading a prepared statement outside Arnold Pistorius's house on Tuesday morning.

Apparently responding to criticism that Pistorius's release after just a year in prison was too lenient, Ms Burgess said the sentence "has not been shortened or reduced".

"He now enters the next phase of his sentence. He will serve this under the strict conditions that govern correctional supervision," she said.

Under South African law, an offender sentenced to five years or less in jail can be released to correctional supervision at home after serving one-sixth of the term - in Pistorius's case 10 months.

The full conditions under which Pistorius must now live for the next four years have not been released by the corrections department and Ms Burgess would not detail them on Tuesday.

Only two conditions for Pistorius's house arrest were previously made public by the correctional services department - Pistorius must continue to undergo psychotherapy while under house arrest and he is not allowed to handle any firearms.

Pistorius would "strictly adhere to" the conditions of his release, Ms Burgess said, and his family would "support him".

Pistorius was acquitted of murder last year for the Valentine's Day shooting death of girlfriend Ms Steenkamp, but prosecutors have appealed the trial verdict of culpable homicide, or manslaughter, and will seek a murder conviction again at South Africa's Supreme Court on November 3.

If Pistorius is convicted of murder by a panel of five judges at the appeal, he faces going back to prison for 15 years, the minimum sentence for murder in South Africa, which no longer has the death penalty.

Pistorius has maintained that he thought Ms Steenkamp was an intruder in his Pretoria home and killed her by mistake. Prosecutors said he shot her intentionally during an argument after she fled to a bathroom stall.

Pistorius, known as "Blade Runner" for his carbon-fiber running blades, gained worldwide fame when he ran against able-bodied athletes at the 2012 London Olympics, the first amputee runner to compete at the games.

Mr Wolela, the correctional services department spokesman, had previously not ruled out ultimately allowing Pistorius to return to training. He also said Pistorius would not be required to wear an electronic tagging device.

Following a suggestion by South Africa's minister of justice that Pistorius should meet with the parents of his slain girlfriend, Tania Koen, the lawyer for Ms Steenkamp's parents, said they would be open to that.

But "the timing would have to be right," Ms Koen said. The proper time would be after Pistorius's case is heard by the Supreme Court, she said, and "he (Pistorius) would have to initiate it".

On the day she was killed by Pistorius, Ms Steenkamp was due to give a speech on domestic abuse at a school.

Her mother will on Wednesday deliver that same speech at a school in the Steenkamps' home town of Port Elizabeth, Ms Koen said.