Oscar Pistorius might not have to spend the rest of his life in prison, but it will be difficult for him to avoid serving at least eight years, experts say.

Judge Thokozile Masipa is preparing to issue her verdict on his murder charges on Thursday night NZT.

Masipa will probably not accept the argument by the prosecution that the South African athlete killed his girlfriend Reeva Steenkamp after planning the act, Cape Town criminal lawyer Keith Gess said.

"But [Pistorius' lawyer Barry] Roux knows his client will not walk away free, so he has been trying to steer the case towards [a verdict of] culpable homicide," the South African term for manslaughter, Gess said. "I expect the verdict will be either murder without premeditation or culpable homicide."

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No date is set for a sentencing, but if Pistorius receives a guilty verdict, Masipa was expected to hand down a sentence within weeks.

"The talk in the corridors is that Pistorius will serve between eight and 12 years," said Stephen Tuson, criminal law and procedure professor at Johannesburg's Witwatersrand University.

The double-amputee Olympic sprinter was on trial from March 3 to August 8, charged with the premeditated murder of Steenkamp, 29, a model whom he shot and fatally wounded through the bathroom door of his Pretoria home in February last year.

Pistorius, 27, said he heard noise from the bathroom and thought a burglar had broken in through the window.

But much of the evidence weighed against the athlete, said criminal law professor James Grant of Witwatersrand University.

If Masipa accepts the prosecution's contention that Pistorius decided to kill Steenkamp after a row, which would amount to premeditated murder, the athlete would face life in prison.

He would be jailed for 25 years before he could apply for parole.

It would not be premeditated if Masipa decides that Pistorius killed Steenkamp knowingly but without planning it or that he fired at a person he thought was an intruder while being aware he was killing a human being.

Pistorius might then face a lengthy jail sentence - up to 25 years - although much would depend on how the court interprets the evidence, Tuson said.

If the athlete is found to have killed Steenkamp without meaning to do so, mistaking her for a burglar and firing without being fully aware of the consequences of his act, he could be sentenced to up to 15 years.

But the sentence for culpable homicide would depend on how callously or negligently Pistorius is deemed to have acted, Tuson said.

If Masipa accepts his argument that he fired in a panic, terrified that an intruder behind the door was threatening his life, he could even be acquitted.

If found guilty of murder without premeditation or culpable homicide, Pistorius would have to serve at least half the sentence before being eligible for parole, Tuson said.

The judge's decision could hinge on testimony by neighbours who said they heard Steenkamp scream before Pistorius fired the fourth and fatal shot, which would indicate he knew she was in the bathroom. "Those witnesses have no reason to lie," Tuson said.

But Masipa has to consider whether they could have misinterpreted the events and weigh the argument by the defence that the screams were from the athlete himself, the professor said.

"The testimonies of the neighbours do not have to be conclusive in themselves, but they can play an important role when woven together with other strands of evidence," Grant said. Those other strands include expert testimony that Steenkamp stood behind the bathroom door wearing street clothes when she was shot, which would indicate she was talking to Pistorius and was prepared to leave his house.

Evidence in Pistorius' favour includes testimony that he was shocked after finding Steenkamp wounded and desperate to save her.

A problem with Pistorius' defence is that it has presented two lines of argument that could be deemed to contradict one another, Grant said.

Roux argued that a terrified Pistorius acted so instinctively that his mind was not in control of his body when he pulled the trigger. But Pistorius' defence also said he believed his life to be in danger, which would indicate he was capable of rational thought.

Masipa will also have to weigh evidence not directly linked to Steenkamp's killing, such as Pistorius' alleged recklessness with guns, his alleged abuse of her and his disability.

Pistorius' legs were amputated below the knee when he was a baby.

Roux argued that the athlete's disability contributed to a heightened insecurity and "fight response", which influenced his behaviour the night of the killing.

But if Masipa accepts the argument of Pistorius' disability being a key factor in the shooting, it would mean "the criteria for reasonable behaviour are not the same for a disabled as for an able-bodied person, which would create a precedent" in South Africa, Tuson said.

- DPA