Former NSW Police detective Gary Jubelin has been found guilty of making illegal recordings during the investigation into missing toddler William Tyrrell.

Key points: A magistrate said Jubelin had no evidence confirming Paul Savage's involvement in William's disappearance

A magistrate said Jubelin had no evidence confirming Paul Savage's involvement in William's disappearance The former detective made no reaction when he learned of the decision in court

The former detective made no reaction when he learned of the decision in court The trial unveiled the investigative techniques used by police to find the toddler

Jubelin, 57, was one of the state's most experienced investigators until he was charged last June, resigning after a career spanning over three decades.

He was accused of breaching the Surveillance Devices Act by illegally recording four conversations with Paul Savage during the Tyrrell investigation, between November 2017 and December 2018.

Police said the recordings were unlawful because they were made on Jubelin's mobile phone and a colleague's, outside the scope of warrants for Mr Savage's home, car and phone to be bugged.

Magistrate Ross Hudson today found the former detective guilty of all four counts and slammed Jubelin for pursuing Mr Savage "at all costs".

"This was above and beyond legality," he said.

William Tyrrell disappeared from the front yard of his foster grandmother's home in 2014. ( Supplied: NSW Police )

Magistrate Hudson said Jubelin belittled and humiliated Mr Savage in lengthy interviews, with a repetition that "may be seen as offensive", despite there being no evidence to confirm his involvement.

"There's no DNA, there's no fingerprints, there's nobody who says 'I saw him go into the backyard', there's no leads, there's nothing," the magistrate said.

"He's picked Mr Savage. He's pursuing him at this stage ... at all costs."

Jubelin always denied wrongdoing, insisting he had "a lawful right and an operational need" to record the material.

But Magistrate Hudson said Jubelin came across in court as "a person playing a role, telling a story", his evidence "self-evidently partisan" and his answers at times "lengthy and self-serving".

Jubelin made no reaction when he learned of the decision in the Downing Centre Local Court.

Outside court, Jubelin said the decision was disappointing.

"Given I've been putting people before the court for the past 32 years, it's important I respect the decision of the court," he said.

"I thought I had a lawful reason to record the conversations. The court has ruled that I don't."

Paul Savage was a person of interest in William's disappearance. ( AAP: Peter Lorimer )

The foster parents of William Tyrrell released a statement after the verdict, saying that Jubelin had "lived and breathed" the investigation.

"The situation is that Gary and his dedicated team members have given everything within their power in their resolve to finding William — even often at great personal expense and loss," the statement read.

The case provided an extraordinary insight into the investigative techniques used to gather information about William's September 2014 disappearance.

The three-year-old was last seen playing in his grandmother's backyard at Kendall, on the Mid North Coast, wearing a Spider-man costume.

Mr Savage, a neighbour, came into the crosshairs of detectives in 2017 because he did not have a solid alibi.

Authorities said his behaviour was "irrational" and he'd previously stalked a post office employee.

In July 2017, police planted a Spider-man suit on a bush track where Mr Savage was known to walk and an undercover officer hid in the bushes to film his reaction.

At another point in the investigation, an undercover officer unsuccessfully attempted to pose as a clairvoyant to exploit the sensitive topic of Mr Savage's late wife.

The case also exposed disharmony within the strike force and Magistrate Hudson concluded there were tensions over who should and shouldn't have been a target.

During his evidence, Jubelin claimed his senior colleague, Detective Superintendence Scott Cook, once said "no-one cares about that little kid" and told him to "get him off the books".

Scott Cook, who was promoted to the rank of Assistant Commissioner in December, denied making the comments.

Mr Savage has denied any involvement in William's disappearance and has never been charged.