NSW cop says foster mum knows where boy is

A senior detective has told a court he believes the former foster mother of missing boy William Tyrrell knows where he is.

The woman appeared in court on Thursday, accused of lying to the NSW Crime Commission.

She cannot be named for legal reasons, but is the former foster mother of missing boy William Tyrrell.

The charges do not relate to William, who was aged three when he went missing from a home at Kendall on the NSW mid-north coast in 2014.

The woman is accused of lying to the commission about whether she had ever struck another child in her care with a wooden spoon.

"I have formed the view (she) knows where William Tyrrell is," Detective Sergeant Andrew Lonergan told Downing Centre Local Court.

Barrister John Stratton SC, defending the woman, said that was a false belief.

Police charged her for allegedly lying, in an attempt to pressure her, he said.

"You are hoping to break her spirit," Mr Stratton suggested.

"Our main objective is to find out where William Tyrrell is," Det Sgt Lonergan said.

Audio recording from listening devices placed in the home recorded what police allege is the woman hitting a child with a wooden spoon.

The child can be heard threatening to call the police beforehand.

A woman is then heard telling the child to "stand up" three times.

"Where'd you put the wooden spoon?" The court heard a woman on the recording say.

The child then pleads, screams and cries, and is told to turn around and move her hands before smacking sounds are heard.

"She's still going on about it," the woman is heard telling her husband, who also cannot be named, in a later phone call intercepted by police.

Det Sgt Lonergan denied he and his colleague Det Sgt Scott Jamieson deliberately lied to the woman to upset her during the interview when they told her police knew where the boy's body was.

Det Sgt Jamieson, who told the woman police knew where William Tyrrell's body was and what happened to him, also denied deliberately lying about it.

"I knew the area in which I believed William was," he told the court.

Both officers said they believed the boy was somewhere in Kendall, however both also acknowledged no body had been found.

The court heard the woman did not admit using a wood spoon, when police asked her about hitting or kicking the other child.

Police prosecutor Amin Assaad submitted she had not said she couldn't remember or was not sure whether she had hit the child with a wooden spoon, but that she said she had "never" done it.

"Unfortunately, that answer ... is an outright lie," he said.

Mr Stratton said she had not been given adequate information.

"No details were given, she was not given the benefit of hearing the tape played," he said.

He submitted it would make no sense for her to admit hitting or kicking the child and then deny it.

She was distressed during the interview and could have made an honest mistake, Mr Stratton suggested.

Det Sgt Lonergan had earlier agreed with Mr Stratton that a mistake is not the same as a lie, after the officer mistakenly gave the wrong date for the expiry date for warrants used to surveil the woman and her husband.

The court will resume on Friday.