Oscar Pistorius has been ordered to undergo psychiatric evaluation.

The paralympian's trial for the murder of girlfriend Reeva Steencamp - who he maintains he shot in the mistaken belief she was an intruder - has been adjourned while he is tested by experts after the judge presiding over the case ruled psychiatric evidence brought before the court in Pretoria could not replace a ''proper inquiry'' into his mental health.

Judge Thokozile Masipa has adjourned the case until next Tuesday (20.05.14), when she will give the details behind a ruling which could see the defendant evaluated for a minimum of 30 days at a psychiatric faculty.

The move comes after forensic psychiatrist Merryll Vorster said earlier this week that the athlete suffered a generalised anxiety disorder (GAD), prompting prosecutor Gerrie Nel to make an application for a psychiatric referral, which the defence team opposed.

However, the prosecutor insisted a referral was necessary because the condition might have affected Pistorius' behaviour on the night he killed his girlfriend.

The judge said the evaluation will ensure Pistorius will get a fair trial, but added it would be ''preferable'' for him to undergo the tests as an outpatient.

Legal experts say the prosecution - who have questioned why the psychiatrist was called to give evidence so late in the proceedings - are keen to show the defence keeps changing the reasons why their client fired his gun, going from putative self-defence, to accidental shooting, and now to something linked to his anxiety disorder.

If Pistorius is found to suffer from a mental illness, he cannot be held criminally responsible for murdering Reeva and would be found not guilty by reason of ''mental illness or intellectual disability''