The woman faced detailed questions in front of the NSW Crime Commission over two days in November 2021 when she was offered a chance to "safely and privately explain" if the toddler suffered a fatal accident in her care.
Under examination at the time, she admitted deleting a text message sent by her partner less than an hour after she claimed she last saw William.
Michael Barnes accepted William Tyrrell's foster mother would not have intentionally harmed him. (Bianca De Marchi/AAP PHOTOS)
Commissioner Michael Barnes told her the purpose of the private hearings was to recover William's body and to allow it to be respectfully interred, allowing loved ones to move on.
"We all accept that you love William and would not have intentionally done him any harm - we also understand that accidents can happen," he told the foster mother.
"If that's what occurred on the day William went missing, this is your chance to safely and privately explain that."
Footage of the interview was played on Thursday to an inquest investigating the three-year-old's disappearance, which has for several days been examining a police theory the foster mother disposed of the toddler's body after he accidentally fell from a balcony.
Police alleged the foster mother's motive for hiding his body was that she feared another child under her care would be taken away.
The three-year-old - who was last seen wearing a Spider-Man costume - went missing while playing on the verandah at his foster grandmother's home in Kendall, on the NSW mid-north coast, on September 12, 2014.
The woman, who cannot be identified, has always denied involvement in his disappearance.
She told the commission that during her frantic search for William, her husband sent a text message after he finished a work meeting to say he would be home in five minutes.
William Tyrrell's foster mother has always denied involvement in his disappearance. (Dan Himbrechts/AAP PHOTOS)
The woman did not call her partner and he only found out his foster son was missing when he arrived at the Kendall property.
"Why didn't you call him at that time to let him know William had disappeared?" counsel assisting the commission Sophie Callan SC asked.
"I don't know, I was just in this panic," the foster mother replied.
She said she didn't call triple zero immediately because she thought the three-year-old could have walked off and been picked up by her partner on the road.
"But you didn't call (him) to check that?" Ms Callan asked.
"No," the foster mother replied.
She said she couldn't remember deleting the text message her partner had sent, but she told the commission it was part of her usual practice of getting rid of inconsequential messages.
The foster mother previously gave evidence about the moments before William vanished.
She went inside to make tea for her mother, hearing the toddler roaring while playing "daddy tiger" outside.
He had also been jumping from the patio and playing a dice game.
Suddenly, there was silence, the foster mother said.
William Tyrrell's biological mother (right) was present when the footage was played in court. (Peter Rae/AAP PHOTOS)
"I say to mum 'that's too quiet' and then I get up and I walk over and I can't see him," she said.
A focus of the examination was a drive she said she took to a nearby riding school while searching for her foster son soon after he vanished.
Police claim she had the opportunity to hide the child's body before she eventually called emergency services.
Footage of that line of questioning is yet to be played to the inquest.
William's foster parents and his biological mother were present at Lidcombe Coroners Court when the footage was played on Thursday.
No one has been charged over William's disappearance and a $1 million reward for information stands.
A decade-long investigation has involved hundreds of persons of interest and dozens of searches.
