Former NSW homicide detective Gary Kevin Jubelin, 57, is accused of illegally recording four conversations in 2017 and 2018 as he investigated how the three-year-old boy went missing from a home in Kendall in September 2014.

Jubelin, who vehemently denies the recordings were illegal, was expecting to have the "full facts" aired in the packed Downing Centre Local Court on Monday, only to hear that material relevant to the proceeding was still subject to a suppression order.

The order, made during the inquest into William's disappearance, also stops the magistrate in the Jubelin hearing from seeing the material in question and can't be adjusted until late on Monday afternoon, the court was told.

The realisation that it would affect the proceeding was only made on Thursday afternoon, the NSW Police commissioner's barrister, Robin Bhalla, said.

"Why you have taken more than two weeks to find that out ... it's despicable," magistrate Ross Hudson said.

"You are stopping someone getting their hearing when they have been waiting since September."

Further delaying the hearing was the fact an assistant commissioner needed to sign a statement about secret evidence, the court heard.

Finding such a senior officer without a conflict of interest to Jubelin was taking time, Mr Bhalla said, describing the roadblock as an "unusual situation".

"(It's) an unusual situation because - I'll be quite frank - the crown solicitor's office has sat on their hands," Mr Hudson replied.

At least one matter under suppression was "fundamental" to the Crown case, prosecutor Philip Hogan said.

A frustrated Jubelin said it had been 12 months since the allegations had been put to him

"We turn up the court today, I was hoping this is where the full facts of the matter come out," he told reporters outside court.

"I'll continue to put my faith in the court."

He said he did not want to comment on the reason behind the delay, only to add he'd "always prided" himself on being "professional and timely".

Eight witnesses are due to be called during the hearing which may now run into next week. The hearing will resume on Tuesday.

Jubelin was accompanied in court by a legion of immediate family, friends, former colleagues and victims' relatives.

Before quitting his post as detective chief inspector, he led a team that re-examined several unsolved and suspected murders, including that of William Tyrrell.

The boy was playing in his foster grandmother's yard at Kendall on the NSW mid-north coast when he vanished in September 2014. He has never been found.

Australian Associated Press