William Tyrrell coronial inquest findings due 2021

The inquest, which has been heard over the last 18 months since March 2019, considered thousands of pieces of evidence collected by Strike Force Rosann, statements by persons of interest and recollections by family and neighbours of the morning little William vanished from Kendall on NSW's Mid North Coast without a trace in 2014.

Overseen by Deputy State Coroner Harriet Grahame, the inquest aims to determine if William's disappearance is a result of misadventure or by the hands of another person or persons.

It concluded in Lidcombe Coroner's Court in Sydney on October 8 with emotional statements by William's foster parents who said the family was "living a nightmare".

His biological parents also presented a short statement to the court saying they are "angry and frustrated" and "want answers".

But it was William's sister who was resolute in her request of the court.

"I hope this speech makes you solve the case," she told the court.

"If it doesn't, when I am officially adult, I will be in the police force, a detective specifically, and I will find my brother and not give up until he is found.

"Please help my family, most of all me, find our precious William."

A search that has not wavered since September 12, 2014 has failed to uncover any trace of the boy, but investigators leading the strike force to find answers believe the truth rests with a predator who abducted him.

The evidence and hundreds of interviews with witnesses and persons of interest to date have built the case that William was taken by someone who now holds a very closely guarded secret that needs to be revealed.

A $1 million reward still stands for information leading to the recovery or return of William. It is one of the largest missing person's reward in the state.

A memory expert told the inquest that the boy's foster mother may have created a "false memory" about suspicious cars in the area at the time of his disappearance.

William, 3, disappeared from his foster grandmother's Kendall home in 2014. He was playing "tigers" with his sister, who cannot be named for legal reasons, and wearing a Spiderman suit.

His foster mother went inside to make a cup of tea, the inquest heard, and when she returned, William was gone.

Forensic psychology academic Helen Paterson told the inquest that information learned after an event could cloud future retelling of events.

"It could be there was a true event ... she saw those cars in that location at a different time," Dr Paterson said.

"But then she mistakenly attributed it to the morning before William disappeared."

Two days after William went missing, the foster mother told police she recalled seeing one white and one grey car parked between two driveways the morning of his disappearance.

She later said the memory of the cars was "burnt into my brain".

"I'm unable to say if it's a true or false memory," Dr Paterson said.

Doubt was also cast over a witness's account about seeing the three-year-old boy standing unrestrained in the back of a four-wheel-drive being driven out of Kendall in September 2014.

The expert was questioned over Kendall resident Ronald Chapman's account of seeing William dressed in a Spiderman suit go by his home on September 12.

Mr Chapman told the inquest in March 2020 that he was "sure" the boy he saw in the back of a Toyota Landcruiser was William Tyrrell.

Dr Paterson said Mr Chapman's retelling of the memory changed over time, with an initial mention of a cape not appearing later and the later accounts told with more confidence and accuracy about key events.

The memory expert said it could be the case Mr Chapman was incorporating "post-event information" such as news reports in his memory reconstruction.

She said her evidence assumed Mr Chapman and the foster mother were giving accounts in good faith.

The inquest heard on October 6 that Herons Creek resident Anna Baker heard a boy's scream coming from bushland shortly after three year old William went missing.

Ms Baker said the scream was "not very long" and sounded like "he was hurt".

"It was a scream and it was silence pretty quickly ... I had no reason to think it was William," she told the inquest.

Her property is situated about four kilometres from Kendall, where William's foster grandmother lived.

She told Deputy Coroner Grahame she was "100 per cent" certain it was a boy's scream.

"I stood up, looked in the direction of the cry and listened," she said.

"But I didn't hear anymore. It was pretty thick bush."

A bid to add high-profile former detective Gary Jubelin to the witness list failed after an application by William's foster parents.

The former detective chief inspector led the police search for William from February 2015 until early 2019.

But Ms Grahame said she already had a significant amount of material based on Jubelin's investigations and he'd responded twice to written requests to present any further evidence he held on the case.

"In my view, there is little that can be gained that is already not contained in the written material," she said.

The move was also resisted by the counsel assisting the coroner Gerard Craddock SC , who said Mr Jubelin wasn't the first or the last detective to lead the investigation.

"The notion it was treated as a missing persons investigation (in the first few months) is just simply wrong - completely and utterly wrong," Mr Craddock said.

"That is just not what happened."

He said the search for William was a criminal investigation "from the get go".

It was perfectly clear Mr Jubelin had opinions, Mr Craddock said, and the detective had been "absolutely dedicated" to finding what happened to William.

New information, not police resources, is what's needed to progress the investigation into William Tyrrell's disappearance, the court heard.

Five investigators are currently assigned to the case, down three from earlier in the year and from a peak of about 26.

NSW Detective Chief Inspector David Laidlaw said he didn't think a bigger reward for information was the option.

"The million-dollar reward, I think, was suitable and appropriate at the time and I still think it is," he said.

"Resources, there's not an issue there at all. I won't shy away from that because the more people that work on it, the better off we will be to hopefully closing the matter.

"But we need that breakthrough of information, (it) is what we really need because to date we haven't found William."

Det Chief Insp Laidlaw said no resources other than information will help.

The coronial inquest findings will be handed down by Deputy State Coroner Grahame on June 18, 2021.

Anyone with information should contact CrimeStoppers on 1800 333 000.