Oscar Pistorius's murder trial has debated his mental state, as the athlete faced a possible stint in a psychiatric hospital to establish if he has a "general anxiety disorder".

Under cross-examination, defence psychiatrist Meryll Vorster told the court that Mr Pistorius's anxiety would have given him a heightened fear of crime.

Mr Pistorius's lawyers have sought to portray the world-famous athlete as almost manically obsessed with safety after a difficult childhood and in the face of high crime levels in South Africa.

The defence wants to show that contributed to Mr Pistorius's reaction when on Valentine's Day last year he allegedly believed his girlfriend to be an intruder and shot her dead through a locked toilet door.

Mr Vorster testified that Mr Pistorius had a "general anxiety disorder" that had an impact on his personal and sexual relationships.

"Did he function socially, yes, one would say he did," she said, "but not optimally."

He took on new friends "in order perhaps to avoid being lonely," said Ms Vorster, "he was simply in their company as to not feel alone".

Earlier in the trial, Mr Pistorius's former friend Darren Fresco and ex-girlfriend Samantha Taylor testified for the state, saying the 27-year-old runner shot a gun out of a moving car's sunroof.

When Mr Fresco finished giving his evidence, he took a seat with Steenkamp supporters in the front row of the public gallery.

Ms Vorster, who spent her second day on the stand, said Mr Pistorius does not fare better in sexual relationships.

"His sexual relationships appear to have been quite short in duration," said Ms Vorster.

Prosecutor Gerrie Nel asked the court to send the sprinter to a facility for 30 days to test Ms Vorster's claim.

Mr Nel is also arguing the Paralympic gold medallist did not exhibit characteristics typical to a person suffering from a heightened general anxiety disorder.

The state prosecutor said Mr Pistorius did not take safety precautions in his house to combat crime, including the absence of burglar bars, a functioning alarm system, and an open bedroom door.

Such negligence did not fit the profile of a person who was abnormally concerned with safety, said Mr Nel.

The star sprinter claims he mistakenly shot girlfriend Reeva Steenkamp through a locked toilet door in the early hours of the morning, believing she was an intruder in his upmarket Pretoria home.