The first week of a coronial inquest to test and hear evidence by Strike Force Rosann investigators is being heard in Sydney NSW Coroner's Court before Deputy State Coroner Harriet Grahame.

The week-long hearing into the child's disappearance and suspected death in September 2014 is likely to find it "was the direct result of human intervention".

"If the evidence establishes that William was abducted, that conclusion is chilling because it means a person snatched a three-year-old from the safety of a quiet village backyard," counsel assisting the coroner, Gerard Craddock SC, said in his opening address on Monday.

"That person, with whatever his or her proclivities and interests, remains in our community."

In a police video played to the inquest, from a backyard on NSW's mid north coast, William's foster father told an officer: "He never wanders. He's not a wanderer."

"I was thinking so many things all at once. I need to cover as much ground as I possibly can in the shortest amount of time," he said.

In a triple-zero call made on September 12 and also played to the inquest, William's foster mother is heard telling the operator: "We heard him roaring around the garden and then I thought, oh I haven't heard him, I better go check on him, and couldn't find him".

The three-year-old boy was wearing a red Spiderman suit and had been roaring like a "daddy tiger" that morning.

The woman had been searching the neighbouring properties and green bushland for any sign of red, and estimated William had disappeared about 10.30am.

In a shaken voice, she also told the operator she hadn't seen anyone suspicious in the area. She provided a description of William stating he was wearing a Spiderman suit.

"We've walked up and down Benaroon Drive and we can't find him," she said.

Mr Craddock said William's foster grandmother - whose house the family were visiting - had noticed "it had become quiet, too quiet".

"There had been one loud roar and then nothing," Mr Craddock said.

Thousands of pieces of evidence have been gathered by Strike Force Rosann, a gun team of police officers and detectives lead by Detective Chief Inspector Gary Jubelin. The team has never given up hope that William will be found alive and returned to Kendall in June 2018 for a forensic search of bushland to help strength their case he was abducted and not a victim of misadventure.

"It's appropriate for us at this particular point in time to conduct this search," Det Ch Insp Jubelin said at the time. What we have done is build up a database of evidence that we readily refer to. The search we are doing and the defined areas we are looking at is for a reason. Everything we do is planned."

Det Ch Insp Jubelin stands by the purpose of the Strike Force's mission - "if you have information, we suggest you come to us before we come to you".

A $1 million reward remains in place for information that leads to the return or recovery of William Tyrrell. It is one of the biggest rewards on offer in NSW

The first week of hearings will explore William's foster and biological families, when he disappeared and early stages of the investigation including the action taken shortly after he went missing.

"I expect the evidence may show that it was likely that he was taken," Mr Craddock said.

"That is, that William's disappearance was the direct result of human intervention."

He said there is "no doubt" both of William's biological parents were in Sydney on the day the young boy disappeared.

"Investigators haven't positively drawn the conclusion that no relative or associate was involved in William's disappearance," he said.

Further hearings will begin in August when persons of interest will be called to give evidence.

Throughout the investigation numerous persons of interest have been identified. Some were charged with unrelated crimes as detectives dug into their pasts, others were cleared entirely.

In 2015 it emerged a ring of pedophiles had been active in the area and were being investigated. Years later, that theory has not conclusively been ruled out. 

Under the powers granted to the coroner, persons of interest may be forced to explain their movements and knowledge of William's disappearance before the court.