After a six-week break, the murder trial that has gripped South Africa and the world resumes on Monday when Paralympian Oscar Pistorius returns to the dock after a month of psychiatric tests.

The star sprinter has tried to argue that a "generalised anxiety disorder" contributed to him shooting dead his 29-year-old model girlfriend Reeva Steenkamp on Valentine's Day last year.

He admits shooting Steenkamp with a 9mm pistol through a locked toilet door, but says it was a mistake as he thought she was an intruder coming to attack him in the dead of night.

The prosecution argues it was cold-blooded murder following a row between the young couple and has gathered evidence of a tempestuous relationship.

Judge Thokozile Masipa last month ordered Pistorius to spend 30 days under psychiatric observation to determine if he should be held criminally responsible for the killing.

The ruling followed testimony from an expert witness for the defence who claimed Pistorius suffers from an anxiety disorder that could explain why he reacted so violently to a perceived break-in.

His defence team has claimed the deep-seated anxiety dates back to the amputation of his lower legs as a child and the influence of a mother who abused alcohol and slept with a gun under her pillow out of fear of South Africa's high crime rates.

The outcome of Pistorius's assessment may dramatically alter the direction of the trial, with leaked media reports suggesting that the three specialists who monitored him had come to an "unanimous" conclusion about his state of mind.

Minor disorder

If they diagnose a serious mental illness, Pistorius may get "admitted to psychiatric hospital indefinitely," said Sean Kaliski, a forensic psychiatrist who conducts hundreds of medical assessments annually at Valkenberg Hospital, a facility outside Cape Town.

But it is highly unlikely that a relatively minor disorder such as a generalised anxiety could have an impact on sentencing in a murder trial, said Kaliski.

"Never, never, no one has ever used it in a forensic sentence. It would be a first if this is used successfully now, a world first," he said.

"It's usually serious matters -- schizophrenia, dementia, bipolar disorder."

Local television network eNCA said on Friday that the three psychiatrists had submitted a brief report to the prosecution and defence, and a full report would be made public once it has been studied by the judge.

Pistorius has been attending daily sessions as an outpatient at Weskoppies Hospital outside Pretoria since May 20.

One of his lawyers, Brian Webber, declined to comment on his well-being, saying: "It's a private matter."

The athlete has often broken down during the proceedings, sobbing and vomiting when graphic details about Steenkamp's death were presented to the court.

His outbursts were so frequent that his defence team started keeping a bucket close by the dock.

His trial started in March and has attracted global media attention. He has pleaded not guilty to Steenkamp's murder and other charges related to ammunition possession.

He faces a maximum of 25 years in prison if convicted of murder.

Judicial sources say once all the evidence has been presented -- estimated to take between one and two weeks -- the defence and prosecution will require a few more weeks to compile their written submissions before presenting them to court.

They will return to court to answer final questions on their arguments. South Africa does not have jury trials, so a verdict will be delivered by the judge after a few weeks' deliberation.