Several persons of interest in the disappearance of NSW boy William Tyrrell - many of whom have never been named - could be forced to reveal what they know under the spotlight of a coronial inquest.
On the fourth anniversary of William's disappearance, NSW Police on Wednesday announced an inquest before Deputy NSW Coroner Harriet Grahame was proposed for early 2019.
A massive brief of evidence will need to be compiled from physical artefacts, thousands of tip-offs and a "persons of interest" list hundreds of names long.
"(An inquest) makes us go over all the evidence collected in the last four years - it's an enormous task," Homicide Squad commander Scott Cook told reporters in Sydney on Wednesday.
"The coroner will consider that and may well ask us to do further things. If that doesn't occur we're likely to see an inquest sometime in the early part of next year."
Detective Superintendent Cook praised the investigators working on the case for their "excellent work".
A police source told AAP detectives working on William's case will push for specific persons of interest - those "at the top of the list" - to give evidence at the inquest.
The coroner's legal powers mean witnesses could be forced to explain their movements and what they know about William's disappearance - unlike conventional police interviews.
Many of the people have never been named in the media, the police source said, adding only "some names" came out publicly during the investigation.
NSW Police in a statement said investigators "would like to acknowledge the continued strength and courage of William Tyrrell's families".
"Over the past year, investigators have continued to explore lines of inquiry in an effort to find out what happened to William, including a large-scale forensic search," it said.
The deputy coroner has requested a brief of evidence which will be provided by the end of the year.
The inquest will be "an opportunity to test information and evidence gathered by Strikeforce Rosann and further the investigation".
"This is another step in ensuring answers are provided to William's loved ones," the police statement said.
William was playing in his grandmother's yard at Kendall on the NSW mid north coast when he vanished on September 12, 2014. He was three.