Potential evidence has been found during the search for missing Sydney boy William Tyrrell.

Police have discovered a toy while searching bushland for new evidence linked to missing Sydney boy William Tyrrell.

The discovery comes on the first day of the fresh search for evidence linked to the boy’s 2014 disappearance.

Although it is too early for police to determine whether the toy belonged to William, it may be used to search for new leads.

The new search will take four weeks and span three square kilometres of bushland in the NSW mid-north coast town of Kendall, where the child was last seen in his foster grandmother’s front yard on September 12, 2014.

The search coincides with William’s seventh birthday, which is in two weeks time.

The group searching the area, which includes experts, riot officers, sniffer dogs and emergency service workers, already covered a large area of land today, finding multiple bags of potential evidence.

On Thursday, the search group is expected to cover a smaller section of land across the road.

Unlike the initial investigation, which focused solely on finding William, this new phase has a different intention.

This time around, police are searching for evidence that William's disappearance was a result of "deliberate human intervention" or foul play.

Detective Chief Inspector Gary Jubelin said, "I don't want this interpreted that the investigation is coming to an end. We are very mindful that it's over 3.5 years since William disappeared and we haven't solved this matter."

"We are going to continue on until we do," he said.

Police strongly believe that there are people out there who know what happened to William, who is known as the boy in the Spiderman suit.

Police are urging anyone with information to come forward.

"You are at a risk of committing a criminal offence by concealing an offence if you don't come forward," Detective Jubelin pleaded.

"I dare say with the intense interest in William's disappearance that someone watching this right now might be feeling very uncomfortable."

If this fresh search doesn't provide new evidence, police will continue to look into the people of interest related to the case.

Listen to The Quicky debrief on the truth about William Tyrrell's parents, and what happened after the three-year-old's disappearance.