An SES volunteer described as "creepy" and who refused to answer detectives' questions, has appeared at an inquest into William Tyrrell's suspected murder.

Key points: Robert Donohoe volunteered for the SES during the search for William Tyrrell at Kendall

Robert Donohoe volunteered for the SES during the search for William Tyrrell at Kendall Not long after he was jailed for sexually assaulting two disabled men

Not long after he was jailed for sexually assaulting two disabled men Mr Donohoe told the inquest his memory was not good after being bashed in jail

Three-year-old William vanished from his foster grandmother's yard at Kendall on the NSW mid-north coast in 2014.

The inquest heard that former petrol station worker Robert Donohoe attended a search zone as a State Emergency Services volunteer in September of that year.

Not long after, he was jailed for sexually assaulting two disabled men.

The inquest was on Friday played a video of a police interview with Mr Donohoe at Wagga Wagga in November last year.

He attended the station with a disability advocate and sat silently as detectives began to ask questions.

Robert Donohoe was among a group of SES volunteers who searched for William Tyrrell. ( ABC News: Jamie McKinnell )

"I'm going to ask you some questions in relation to the disappearance and suspected murder of William Tyrrell on the 12th day of September 2014," Detective Senior Sergeant Mark Dukes asked.

Mr Donohoe did not respond to the questions.

During the video, Detective Senior Sergeant Dukes acknowledged Mr Donohoe had been given legal advice not to say anything.

At the beginning of his appearance before the inquest, Mr Donohoe warned that his memory was not good and he had a cognitive impairment.

"I've been bashed in jail and everything, so my memory's not 100 per cent," he told the court.

He recalled the search zone being "like a football match" because it was so busy, saying there was nowhere to park and there was a strong media presence.

William Tyrrell went missing in 2014. ( Supplied: NSW Police )

The inquest has previously heard from Mr Donohoe's then-boss, Sharon Starr, who described him as "creepy".

She said Mr Donohoe would often keep chickens in his van, which was later seized by detectives.

Mr Donohoe was excused as a witness but may be recalled at a later stage.

Coroner says former detective cannot give evidence

Peter O'Brien, a lawyer acting for Bill Spedding, a person of interest in the investigation, made an application on Friday to add former detective Gary Jubelin and other senior police who led the investigation to the witness list.

The former detective was stood down from the investigation over allegations he unlawfully recorded conversations without a warrant and has pleaded not guilty.

Gary Jubelin was stood down from the investigation. ( AAP: Joel Carrett )

Mr O'Brien described the investigation as "frustrating and damaging" for his client.

"He and the rest of the community are entitled to know whether it could have been done better," he said.

But Deputy state coroner Harriet Grahame rejected the application and said her "primary focus and greatest hope" was to find out what happened to three-year-old William.

"It is not the time for trying to discover, on a piecemeal basis, if the investigation went down a wrong track," she said.

"If there is a need to look at the adequacy of the investigation, it is not now."

Ms Grahame said calling the senior officers would be "unwieldy" and would cause the inquest to lose focus.

The inquest continues.