"Whoever has him needs a bullet": William Tyrrell's biological mother speaks for the first time.

Three-year-old William Tyrrell was playing “hide and seek” with his sister in the yard of his grandmother’s house when he disappeared in the middle of the day on September 12, 2014.

His foster mother and grandmother were sitting outside at the Kendall property on the New South Wales mid-north coast watching the pair, when his mother went inside to make a cup of tea.

The last thing she remembers hearing from William, who was wearing a Spiderman suit at the time of his disappearance, is a tiger’s “roooaaarrr” as he was running towards the house. Then, silence.

Listen to The Quicky debrief on the truth about William Tyrrell’s parents, and what happened after the three-year-old’s disappearance. Post continues below.

The identities of William’s foster family have not been made public. But now, for the first time since the little boy’s disappearance three-and-a-half years ago, William’s biological mother has spoken to the media.

“I felt like I was the worst mum in the world,” Karlie Tyrrell tells Channel Seven’s Melissa Doyle in an exclusive interview to be aired on Sunday night.

Watch the program’s preview here:

Karlie has a string of petty crimes to her name, including drug offences and assaulting a police officer, Daily Mail reports. William’s biological father is currently serving jail time for stealing.

William was taken from Karlie as a toddler and fostered to a family from Sydney’s Upper North Shore. But the fact William was under foster care when he disappeared was only made public in August last year after the New South Wales Supreme Court of Appeal ruled it a matter “of legitimate public interest”.

During the Channel Seven preview, Karlie can be heard saying "they were responsible, they failed", likely a reference to either William's foster family or the Department of Family and Community Services.

No trace of William has been found since police began searching the Kendall property and surrounding woods on the night of his disappearance.

In 2015, police told News Corp they had "information that could link William's disappearance to a group of people suspected of paedophile activity", but nothing has been confirmed.

"Whoever has him needs a bullet," Karlie says in the preview for the  Sunday Night episode.

The clip shows Karlie anguished and holding back tears, particularly after Doyle's question regarding any possible involvement Kylie might have had in the disappearance of her son.

The woman's answer to the question has not yet been aired. However, according to News Corp, both William's biological family and his foster family have been cleared by police for any involvement in his disappearance.

The place William disappeared is the outskirts of a quiet, friendly town on the NSW mid-north coast. It's a community in which the neighbours know each other, and many were involved in the search for William in the days following his disappearance.

"If he had wandered, he would have been found," neighbour Paul Savage told Newcastle Herald in January, 2015.

The property where William was playing "hide and seek" sits on top of a hill, overlooking the only road leading to and from the house.

The lot is surrounded by thick shrub and how the three-year-old could have been lured from the property - either via the road, or one of two steep walking tracks through the bush - remains a mystery.

"It is out of town and out of the way. Who would be driving there on that Friday morning at that time?" NSW police Super Indendent Paul Fehon told Newcastle Herald.

"It is a dead-end street."

Holly Wainwright and The Stephens Twins explore our fascination with true crime. Post continues after audio.

Police have not given up hope finding William alive, with police telling News Corp in August last year: "The investigation into the disappearance of William Tyrrell is very much ongoing."

"We are reminding the community that a young child is still missing and our focus will always be on bringing William home."

If he is alive, William will be turning seven in June.

The full episode with Karlie Tyrrell will air on Channel Seven's Sunday Night program, this Sunday at 8:30.