Former head of the William Tyrrell investigation says he says he thinks he knows who took the boy

William, who has been missing since September 12, 2014, would be aged eight if alive today. Photo / News Corp Australia

The chief homicide detective who investigated the 2014 disappearance of three-year-old William Tyrrell says he has an idea of who was responsible.

Speaking at a crime writer's festival in Sydney yesterday, Gary Jubelin said he could not be "100 per cent certain" but added: "I have my thoughts, not just gut instinct."

William went missing from his foster grandmother's home in Kendall on NSW's Mid North Coast about 10.15am on September 12 in 2014.

A coronial inquiry into the boy's disappearance continues and Mr Jubelin said he had faith in the process, news.com.au reports.

"I have thoughts, ideas about what may have happened, but I want to stress that there is a process with the coroner and an ongoing inquest which I respect totally and support," he said.

Mr Jubelin was taken off the investigation after he was accused of professional misconduct related to his handling of the case.

He told the audience in Sydney yesterday he regarded his inability to solve the case as a "personal failure".

Mr Jubelin retired from the police force last month after he was charged with illegally recording four conversations during the investigation. He has pleaded not guilty to those charges.

"Those charges relate specifically to me carrying out my duties. It was within the scope of what I was tasked to do, investigating the William Tyrrell matter," he said.

"I had a lawful reason to record those conversations and an operational need.

"It has impacted on me greatly. Obviously, I've left the police. I am in the police to support victims and lock up bad guys. And I wasn't able to do that."

Half-joking, he told the audience in Sydney his main aspiration now was to stay out of prison.

"If I go to jail, I'm sure I'm going to be very popular," he said.

Mr Jubelin's case is listed for hearing on September 24.

His public appearance comes as a new person of interest has been revealed in the William Tyrrell case.

The person who once lived at a sawmill that was searched a fortnight ago will give evidence at the inquest next year.

Frank Abbott is expected to testify at the NSW inquest into the suspected abduction of three-year-old William when it resumes in March 2020, a source has told AAP.

Mr Abbott was living in a caravan on a sawmill near Kendall when the boy disappeared five years ago, Ten News reported on Wednesday.

Police, sniffer dogs and SES personnel searched the sawmill on Herons Creek Road two weeks ago as the inquest was sitting in Taree.

Heavy machinery was used to move logs during the search, with police tight-lipped at the time as to why they were interested in the property.

No one has ever been charged in relation to William's disappearance.

Hundreds of people have been dubbed "persons of interest" during the investigation, and a case detective admitted during the inquest there was "a very low standard to meet in order to become a person of interest".

Counsel assisting the coroner Gerard Craddock SC in early August stressed that any suggestion those called to give evidence were suspects was "simply wrong".

"This is an inquest and not a criminal trial," he said.