The three-year-old went missing while playing at his foster grandmother's home in Kendall, on the NSW mid-north coast, on September 12, 2014.
On Monday, counsel assisting Gerard Craddock SC reopened the inquest for a fifth round of evidence, saying the focus would be on a theory the foster mother was involved in unlawfully disposing of William's body after his accidental death.
Investigators allege William's foster mother disposed of his body after his accidental death. (HANDOUT/NSW POLICE)
After the three-year-old died at the Kendall property, police believe the foster mum might have loaded his body into her own mother's Mazda before driving down the road to dispose of his remains, deputy NSW coroner Harriet Grahame heard.
It was only then that she called triple zero, according to investigators' theory.
"Police assert that she must have quickly resolved that if the accidental death were to be discovered, she might lose (her other foster child)," Mr Craddock said.
The foster mother has always denied having anything to do with William's disappearance.
Evidence will be given from a truck driver who drove past where the foster mother allegedly disposed of the body at the time.
The inquest into William's disappearance began in March 2019 but was adjourned in October 2020.
No one has been charged in the case and a $1 million reward for information still stands.
Despite a decade-long investigation involving hundreds of persons of interest and dozens of searches, no trace of the boy - last seen wearing a Spider-Man suit - has been found.
In November, 2021 police launched a fresh, month-long search for William's remains that concluded without any obvious breakthroughs.
A 2021 search for William Tyrrell's remains was more intensive, the inquest was told. (Mick Tsikas/AAP PHOTOS)
This round of the inquest will examine a 2021 search of the Kendall property and surrounds - the third by police into William's whereabouts.
Mr Craddock said after an initial search for the toddler in September 2014, further sweeps of the area using GPS trackers and police dogs were also conducted in 2018 and 2021.
"The 2018 forensic search was an intensive, thorough search," he said.Â
"By comparison, the 2021 search was at a further level of intensity."
In looking for William, police followed through on tip-offs from prison inmates and clairvoyants claiming to have information.
One lead involved a car that was heard doing a U-turn in the street outside the foster grandmother's home about the time of the disappearance, Mr Craddock said.
But police were unable to verify that information or track down the vehicle.
The inquest continues.
