The former NSW Police detective chief inspector led the police investigation from early 2015 until March 2019, when investigations began into illegal recordings he'd made while interviewing a person of interest.

William has not been seen since disappearing from his foster grandmother's home in Kendall, on the NSW mid-north coast on the morning of September 12, 2014.

The inquest, resuming on Tuesday in Sydney, heard the foster family believed Mr Jubelin could shed light on particular lines of investigation over the years and what lines of inquiry were still outstanding in March 2019.

"Concerns are held by our clients about what time ... a decision was made to change the investigation and escalate it ... as a possible abduction or possible homicide of William," Justine Hopper, the family's barrister, told the NSW Coroners Court.

But the move was resisted by the counsel assisting the coroner, who said Mr Jubelin wasn't the first or the last detective to lead the investigation.

"The notion it was treated as a missing persons investigation (in the first few months) is just simply wrong - completely and utterly wrong," counsel assisting Gerard Craddock SC said.

"That is just not what happened."

He said the search for William was a criminal investigation "from the get go".

It was perfectly clear Mr Jubelin had opinions, Mr Craddock said, and the detective had been "absolutely dedicated" to finding what happened to William

"But Your Honour isn't missing anything because he's not on the investigation anymore," he submitted.

Coroner Harriet Grahame is expected to make a decision later on Tuesday.

Beginning in March 2019, the inquest has been heard in several stages in Sydney and Taree.

It was due to finish in March 2020 but abruptly halted due to the coronavirus pandemic.

Australian Associated Press