A top NSW detective has been taken off the investigation into missing boy William Tyrrell over alleged misconduct claims.
An internal investigation has begun into Detective Chief Inspector Gary Jubelin who last year led a fresh search for William, a NSW Police spokesman a NSW Police spokesman told AAP on Thursday.
It's understood Detective Inspector Jubelin is being interviewed by the Professional Standards Command over his conduct on Strike Force Rosann, which was set up to investigate the three-year-old's disappearance.
He's facing allegations relating to staff management and using a mobile phone without a warrant to record someone, it is understood.
An inquest into William's 2014 disappearance is due to be held in March and will run for a week before resuming in August.
In the fourth year since William's disappearance, the inquest will be held before Deputy NSW Coroner Harriet Grahame.
"The acting state coroner and counsel assisting have been briefed, and the William Tyrrell inquest will proceed as planned," police said.
At a directions hearing held before the State Coroner's Court in December, Counsel assisting the coroner, Gerard Craddock SC, said it wasn't presently possible to conclude William was dead.
"The police investigation into his disappearance is ongoing and police are following active leads at present," Mr Craddock said.
The inquest is currently scheduled to begin on March 25 with the first segment to run for five days.
A massive brief of evidence has been compiled from physical artifacts, thousands of tip-offs and a "persons of interest" list hundreds of names long.
More than 15,000 pieces of information have been collected over the last four years.
"(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 said.
"The coroner will consider that and may well ask us to do further things.”
Detective Chief Inspector Gary Jubelin, head of Strike Force Rosann since the start of the search for the missing child in 2014, sent a search team back into bushland at Cedars Loggers Lane and Batar Creek Road in Batar Creek on June 27, 2018, confirming this was a new specific area of interest.
He is known for his dogged determination to find out what happened to the little boy.
“We strongly believe there are people out there who have information on this and I make a point to those people – if you do have information concerning what happened to William you are at risk of committing a criminal offence by concealing an offence if you do not come forward,” Det Insp Jubelin said at the search site in Kendall last year.
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.
A $1 million reward, one of the highest ever offered in NSW, still stands for information leading to the return or recovery of William Tyrrell.
- Dominica Sanda, AAP