William Tyrrell's biological father admits he was "the number one person not letting him go" when he hid the boy from authorities for six weeks when he was a baby.
But he denies playing any part in his son's disappearance two-and-a-half years later.
The child was aged three years and three months when he vanished without a trace from the backyard of his foster grandmother's house in Kendall on the NSW mid north coast on September 12, 2014.
William's biological parents, who can't be identified and are no longer in a relationship, were the final witnesses to give evidence on Thursday at the first week of the inquest into his disappearance and suspected death.
When questioned about the involvement of authorities, including the Department of Family and Community Services, William's father said: "They f***ed up."
"It was the minister's duty of care to keep him safe until he was 18 and that was not the case at all. FACS was about keeping kids safe."
Both parents conceded they "absconded" with William for five to six weeks in early 2012 after a children's court ordered he be handed to FACS and placed in foster care.
"I couldn't bring myself to give them my son," the mother told the NSW Coroners Court.
The father said he was upset, annoyed and angry.
"I was the number one person not letting him go. The number one person putting that (idea) forward," he said.
The mother said her last supervised contact visit with William was in August 2014.
She first found out William was missing when police knocked on their door in Sydney hours after he vanished.
"They didn't tell me anything. They asked me if William was there. They looked around, they thought (my other) son was William - he wasn't. They asked what I'd done ... and left," she said.
She still stands by her statement to police that she "didn't take him" and "definitely" doesn't know where he is.
In that document, the woman stated: "If I took him, I would be gone and I would have (his sister) as well. I want a normal life. I don't want to be hiding away with them somewhere."
The mother told the court she was aware of adoption plans before William disappeared but not in any detail and "didn't agree" with them.
"We were still trying to get the children back. We were at court," she said.
In closing the first week of the inquest on Thursday, senior counsel assisting the coroner, Gerard Craddock SC, said the police investigation continued.
"We certainly haven't given up. If anybody, either in the room or in the community generally has any further information which they think might be of assistance ... we want to hear from them," he said.
Mr Craddock and Deputy State Coroner Harriet Grahame agreed the past week had been "the tip of the iceberg".
A directions hearing will be held on April 24 before four weeks of hearings in August when persons of interest will be called to give evidence.
Australian Associated Press
