William Tyrrell's biological mum breaks silence on his disappearance

The biological mum of missing NSW child William Tyrrell has spoken for the first time about her son's disappearance three-and-a-half years ago.

Speaking to Channel 7's Sunday Night, Karlie Tyrrell denied any role in the little boy's disappearance, saying she still believes her son is alive, and pleading to anyone holding William to "let him come home".

The three-year-old was playing in his foster grandmother's garden in Kendall on the NSW coast in September 2014 when he vanished, sparking the biggest manhunt in Australian history.

Karlie, now 29 and a mum of four, pointed the finger at his foster mother and foster grandmother for negligence.

"They were responsible for looking after him, and they failed," she said.

She described the knock at the door from police officers looking for her little boy that day.

Eight months pregnant, she said she had just returned from a shopping trip in Sydney to buy baby supplies.

"Police were knocking on the door asking where William was. I was in shock," she said.

"They asked if William was here ... William’s not even in my care. I don’t know why you’re coming to ask me.

'They could have looked a bit better. Not that I did have him, but their police skills were not very high standards."

Reporting restrictions prevented the media from revealing her identity in the immediate aftermath of William's disappearance. He was taken in to care when he was eight months old.

Karlie said she locked herself away because she felt like "the worst mother in the world".

"Because I haven't come out and said anything, of course, people are gonna assume I don't care.

"And that's OK, because I know that I do.

"Their judgement doesn't bother me, really. I'm not a bad person."

Karlie also talked openly about her children being taken in to care, saying she's "not a bad mum".

"Because I had domestic violence and drugs and alcohol, marijuana. When I had to do a drug screen, I tested positive after I'd had my children."

She then pleaded with her son's potential kidnapper not to "hurt him".

"Just let home come home. He hasn’t even met his little brother yet. That’s not fair, it’s just not fair," she said.

Karlie recently fronted court after pleading guilty to spitting in a police officer's face during an altercation at a Sydney shopping centre. She will be sentenced this month.