Oscar Pistorius has been jailed for five years for killing his girlfriend, Reeva Steenkamp, on Valentine's Day last year.
His defence team said he would spend a sixth of the term in prison - 10 months - before being placed under house arrest for the remainder of his sentence.
But Nathi Mncube, a spokesperson for the National Prosecuting Authority, said he was likely to serve one third of the sentence behind bars, before adding "it's not for the NPA to decide".
"This is not what we were hoping to achieve at the end of this trial ... but it is not entirely up to us," he told Sky News.
The 27-year-old, who was found guilty of culpable homicide last month, appeared to wipe his eyes as the sentence was handed down.
Judge Thokozile Masipa had ruled the Olympic and Paralympic athlete did not intend to kill his girlfriend when he fired through the bathroom door at his home in Pretoria.
"Having regard to the circumstances in the matter, I am of the view that a non-custodial sentence would send the wrong message to the community," she said.
"On the other hand, a long sentence would also not be appropriate either, as it would lack the element of mercy.
"What may appear to be justice to the general public may not be justice. Society cannot always get what they want."
She said Pistorius' skill using a gun was an aggravating factor but that she also recognised his remorse.
She said he "fired not one shot but four shots into the door" of a small cubicle "with no room for escape".
He was also handed a suspended three-year prison sentence for the discharging of a firearm, over an incident in which a gun accidentally went off in a restaurant.
Pistorius is expected to spend his jail time in the hospital wing of Pretoria's prison.
Ms Steenkamp's father, Barry, said he was "satisfied" with the outcome.
Her mother, June Steenkamp, said the sentence brought a sort of closure, but that full closure was not possible "unless you can magic her back".
Sentencing options available to the judge in the culpable homicide case included up to 15 years in jail, a suspended sentence, a correctional supervision or fine.
It brings to a close a seven-month trial, which attracted worldwide attention.
After one of the most sensational court cases in the country's history, the sentence is expected to fuel further controversy about race and money in its justice system.
Pistorius' legal team argued during the trial that he mistook Ms Steenkamp for an intruder and believed they were both in danger.
His lawyer Barry Roux said the athlete had "lost everything" after the 29-year-old law graduate and model's death, and argued he should not go to prison.
He also said the Pistorius, whose lower legs were amputated as a baby, would face particular difficulties in jail.
But state prosecutor Gerrie Nel had argued that only 10 years' imprisonment would satisfy the public.
He said that any lesser sentence could result in a widespread loss of faith in the justice system.
Known as Blade Runner because of his carbon-fibre prosthetics, Pistorius achieved global recognition at the London 2012 Olympics when he reached the semi-finals of the 400m against able-bodied athletes.