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NSW police seek forensic Tyrrell evidence

Police are combing through dense bushland on NSW's mid-north coast, trying to find evidence to finally explain what happened to William Tyrrell.

Investigators are hoping to rule out an accident and confirm suspicions he was abducted from his grandmother's yard in Kendall on September 12, 2014, aged three.

Detective Chief Inspector Gary Jubelin said now was simply the "appropriate" time for a detailed search of the area.

The initial search focused only on finding "a little boy lost" across about 18 square kilometres.

The new search will home in on a three square kilometre zone, and every detail will be documented.

"So we can show, beyond reasonable doubt, that William's disappearance was the result of human intervention and not through misadventure," Det Ch Insp Jubelin told reporters at the search site on Wednesday.

If police manage to lay charges against one of many persons of interest, anything they find could become evidence in a criminal court case.

If they find nothing, and the rest of the investigation falls flat, the search information will be handed over to the coroner for an inquest.

About 50 officers from the public order and riot squad spent Wednesday raking through about 600 square metres of bush near Kendall with picks, hoes and shovels. Sniffer dogs were also deployed and a chainsaw on hand.

Some officers were seen leaving the dense scrub carrying evidence bags of "foreign objects" but nothing believed to be linked to the case.

Det Insp Jubelin said time was no barrier with forensic evidence, but he held "grave, grave concerns" about the boy.

"It has been a very long time. But until we know conclusively that William is not alive we'll treat it with the possibility that he is still alive."

He wouldn't confirm if specific information led to the fresh search.

Hundreds of people remain on the persons of interest list, but few were considered high priority, he said.

"I want to stress we have numerous lines of inquiry including persons of interest that we are going to fully exhaust before the matter goes to a coroner," he said.

Det Insp Jubelin believes people do know what happened to the boy and reiterated the $1 million reward for information leading to William's recovery.

William's foster family have never given up hope of finding the boy, and on Wednesday posted a message to social media.

"Today marks 1370 heartbreaking days since William disappeared," it read, ending with: "He is in our hearts always ... Always".

The search will resume on Thursday morning.

© AAP 2018