Police set up a joint investigation into the disappearance of William Tyrrell so they could use a secretive commission's powers to coerce information.
One of the missing toddler's former foster parents has previously been acquitted of lying to that commission and the other has pleaded not guilty to similar charges.
The NSW Police officer-in-charge, Detective Sergeant Andrew Lonergan, told a court on Monday no members of the NSW Crime Commission were seconded to the homicide team as part of the investigation.
Phillip English, a barrister for one of William's former foster parents, neither of whom can be identified for legal reasons, asked Det Sgt Lonergan if the only reason to set up the joint investigation was to use the commission's coercive powers.
"Correct," Det Sgt Lonergan told the Downing Centre Local Court on Monday.
The court heard police had the former foster parents under surveillance for almost an entire year from December 2020 onwards.
The foster parents were called before the NSW Crime Commission in November 2021, where some recordings from those surveillance devices were played.
The former foster father has pleaded not guilty to five counts of knowingly giving false or misleading evidence at a hearing, relating to answers he gave the commission.
The foster mother was acquitted in November 2022.
The questions related to a "propensity" to harm children in their care, Det Sgt Lonergan said.
Mr English said prosecutors need to prove that the foster father gave false evidence and did so knowing it was false.
Reading transcripts of the hearing with Det Sgt Lonergan in the witness box, Mr English said his client had readily acknowledged he may have been incorrect when presented with contradictory information.
The foster mother pleaded guilty in September to assaulting a child in their care, who is not William, kicking them and hitting them with a wooden spoon.
"I hit (the child) really hard with that wooden spoon," the foster mother said in a recorded phone call to the foster father, played to the court on Monday.
Det Sgt Lonergan confirmed a brief has been served to prosecutors recommending charges of perverting the course of justice and interfering with a corpse.
Reports of the brief surfaced in June, prompting a response from the foster mother's lawyer who called for detectives to disclose evidence.
"The foster mother has always, and maintains, she has nothing to do with William's disappearance," read a statement from her lawyer in June.
Three-year-old William vanished from a Kendall property belonging to his foster grandmother on the NSW mid-north coast in 2014, and remains one of Australia's most high-profile missing person cases.
"We simply don't know what happened to him that day," Det Sgt Lonergan told the court on Monday.
He said the former foster father had been eliminated from suspicion of having anything to do with William's disappearance, "to date".
Both foster parents deny any involvement.
Another man pursued as a suspect by investigators was awarded $1.8 million in August following an unsuccessful appeal against a NSW Supreme Court decision that he was maliciously prosecuted on other charges, which described police allegations against him as "concocted and false".
The court hearing resumes on Tuesday.
