About 50 specialised officers marched into the scrub about 8am on Thursday, joined by SES and Rural Fire Service members.

The terrain area to be scoured is much steeper and denser than Wednesday's section so emergency workers headed in first to cut up fallen trees and clear the way for forensic teams.

Day one of the fresh search on Wednesday covered about 600 square metres of Kendall bushland near the site of the boy's 2014 disappearance, with forensic teams combing the land with picks, hoes and shovels looking for evidence.

Among the foreign objects removed and catalogued was a child's toy - though it is not yet believed to be relevant to the investigation.

The objects will join the already massive bank of evidence amassed for the case - including 15,000 pieces of information and hundreds of persons of interest.

Over the next four weeks, police will comb 16 sections of bushland, going sector by sector.

The NSW Coroner's office was advised of the fresh search, announced earlier this week, and investigators expect a visit from the office in the coming weeks, AAP understands.

Detective Chief Inspector Gary Jubelin, on Wednesday, did not rule out the investigation would become a coronial inquest - but told reporters in Kendall there were many leads for police to exhaust before then.

He said the new search would focus on collecting forensic evidence from the bush, whereas the original search focused simply on finding "a little boy lost".

"Until we know conclusively, that William is not alive, we'll treat it with the possibility that he still is alive," he said.

"But obviously we have grave concerns."

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

Residents in the quiet Kendall street, where a large police presence and media contingency has set up, are keeping an eye on activity from the verandahs of their homes while getting on with day-to-day life.

Many are reluctant to talk to media.

Australian Associated Press