"Blade Runner" Oscar Pistorius was not wearing his prosthetic legs when he tried to force open a locked toilet door in his house on the night he shot his girlfriend, a forensic expert told his murder trial on Wednesday.

Colonel Johannes Vermeulen's testimony contradicted Pistorius's account of how he had broken down the door after firing four shots into it, fatally injuring Reeva Steenkamp.

Vermeulen told the High Court in Pretoria the marks on the wooden door were made by an autographed cricket bat belonging to Pistorius, but were considerably lower than they would have been had the disabled athlete worn his prostheses.

"[I] mentioned that marks on the door are actually consistent with him not having his legs on and I suspect it must be similar to the height that he was when he fired the first shots," he said under questioning from State prosecutor Gerrie Nel.

At Nel's request, Vermeulen gave the court a demonstration of how the accused would have wielded the bat -- exhibit one in the murder trial --to leave the marks it did on the door.

He showed the court that he could not comfortably stand up straight to strike the door at the required angle, and then bent down to strike the door where it bore a dent made on the night of the shooting, 1.53m from the bottom.

"It's quite an uncomfortable position," a stooped Vermeulen commented.

Nel asked that Vermeulen hold the position and a tape measure be brought to record his shoulder height.

It came to around 1.25 metres from the court room floor.

Vermeulen told the court that he had 29 years of experience in forensic work, but conceded that he was not called to Pistorius's luxury townhouse in Pretoria -- the scene of the shooting -- for some time.

He was only given the door -- which was displayed in court on Wednesday fitted to a frame and replica of the toilet cubicle -- on March 8 last year, more than two weeks after Steenkamp was shot in the early hours of Valentine's Day.

Pistorius was released on bail in late February, and at the bail hearing the State had proceeded from the assumption that the paralympian athlete had worn his prosthesis.

This, Nel made plain on Wednesday, was not the State's argument in trial.

He rose to correct a statement by Oscar Pistorius's lawyer Barry Roux, who said: "The State's case is that he was on his prostheses."

Nel rose from his seat and objected.

"It's not my case...That was perhaps something that happened in the bail application," he said.

Pistorius himself told the bail hearing that when he shot at the toilet door, he was on his stumps and scared, thinking that there was an intruder hiding inside the cubicle.

He said he subsequently realised that Steenkamp was not in bed and after putting on his prostheses, first tried to kick open the door, then bashed at it with the cricket bat.

"I put on my prosthetic legs, ran back to the bathroom and tried to kick the toilet door open," the double amputee who became an Olympic and paralympic sprinter told the bail hearing.

"I think I must then have turned on the lights. I went back into the bedroom and grabbed my cricket bat to bash open the toilet door."

After Nel concluded questioning of the forensic expert, Roux said he had not been forewarned of his testimony and asked for a tea break to examine the door and the marks.

Cross-examining Vermeulen afterwards, he asked why he never made a report with his findings available to the defence team and suggested he felt "ambushed".

He went on to ask Vermeulen to return to the door and then kneel, lift his feet off the floor and strike at it with the cricket bat.

"Mr Pistorius has even less support," he remarked when Vermeulen said it was an uncomfortable position.

But Vermeulen rejected the lawyer's suggestion that Pistorius would therefore not have been able to balance well enough on his stumps to break the door.

He said Pistorius had managed to balance on his stumps while firing a gun at the door, and given how the amputee had adapted to his disability, it can be assumed that he would have been able to balance while swinging a cricket bat.

Pistorius made notes while listening to the testimony. He has pleaded not guilty to premeditated murder and earlier this week became violently ill in court while listening to clinical detail of Steenkamp's wounds.

His neighbours have testified that they heard a woman's terrified screams ring out from the direction of his home, followed by rapid gunfire. But Pistorius has rejected the State's submission that he argued with Steenkamp on the night of her death as "unfair and incorrect".