Police comb bushland for missing Aussie boy William Tyrell, nearly four years after he disappeared

Detectives are hoping a forensic search of bushland on NSW's mid north coast will rule out if William Tyrrell became lost and confirm suspicions he was abducted.

William was three years old when he vanished from his grandmother's yard in Kendall on September 12, 2014.

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 sq/km zone, and every detail will be documented.

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

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.

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

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

The search was set to resume this morning.

William Tyrell
William Tyrell Source: NSW police