'They were responsible': William Tyrrell's mum says foster family 'failed' him

The mother of missing child William Tyrrell has broken her silence about her son's abduction for the first time, saying she believes his foster family "failed" to look after him.

Karlie Tyrrell has never spoken publicly about William, who was just four years old when he vanished from his foster grandmother's home in Kendall, NSW, in 2014.

"I don't want to blame the carers, but they were responsible for looking after him and they failed", Ms Tyrrell said.

Mum Karlie Tyrrell has spoken out about her son's disappearance. Source; Sunday Night

William was nine-months-old when he joined his older sister to live with his foster parents, a professional couple living in Sydney's leafy north shore.

"I just made bad choices and I wasn't ready I guess to have my daughter", an emotional Ms Tyrrell said.

"I had domestic violence and drugs and alcohol, marijuana".

"When I had to do a drug screen I tested positive after I had my children".

Despite her past addictions, Ms Tyrrell, a mum-of-four, insists she's not a bad mother.

"Because I haven't come out and said anything, of course people are going to assume I don't care and that's okay because I know that I do", she said.

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

Ms Tyrrell said having two of her children taken into foster care was a deeply traumatic experience as a young mother.

Her daughter is still with the same family that William was abducted from.

"I am still trying to do everything I can, but I don't know what else to do", she said.

"I didn't want to go on news being angry or anything, making anything worse, it didn't mean I didn't love him".

A million-dollar reward is still on offer for information that leads police to William, who will be seven in June.

Please contact Crime Stoppers on 1800 333 000 if you know anything about the disappearance.