William's family have made a heartfelt plea to the community about their desperation for the little boy.
Friend of the family Nicole addressed the media on Sunday afternoon at the Kendall Showgrounds.
"William is a much-loved and cherished little boy," Nicole said. "The family are desperate to have him home, and if anyone knows anything please contact police."
Superintendent Paul Fehon said the search will continue on Monday if needed, with extra resources due to arrive from Sydney.
Superintendent Fehon also said the search will continue overnight, but admitted less was possible.
"We always have a search capability, but in darkness there are limits to some of that capability," Superintendent Fehon said.
The region's top cop said William's chance of survival is steadily decreasing.
"As time passes that window does diminish."
Sun, 1.50pm: Fine conditions are expected to continue during the afternoon and night as people continue to register in large numbers at the Kendall Showgrounds.
Weatherzone said there is a chance of a light shower in the early part of the evening, but light winds and sunshine seem likely as the search grows.
Police said William's sister who was playing with him prior to his disappearance has been interviewed, but was unable to provide any fresh information.
Inspector Kim Fehon said investigators are "looking into other possible scenarios" for the child's disappearance.
Sun, 10.15am: Police have said today is "incredibly critical" to find William Tyrell, after doctors said the little boy must now be in a poor state of health.
Inspector Kim Fehon made the announcement as she confirmed aspects of the search would change on Sunday.
PolAir are on standby after providing extensive aerial coverage for the first two days.
The Dog Squad have also been withdrawn, but police divers have arrived to bolster the ranks.
"All dams within any possibly radius will be searched by end of the day," Inspector Fehon said.
The family of the missing boy were very thankful for the community's staggering response so far, police said.
The SES have told police they currently have sufficient resources, but Inspector Fehon said more will be provided if needed.
7am: A FULL-SCALE search is continuing on Sunday morning for three year old Williams Tyrell in bushland around Kendall.
Inspector Kim Fehon addressed volunteers at the Kendall Showground and stressed the need to ensure 'every blade of grass' was checked to find the little boy.
"Today is an important day," the Inspector told the hundreds of volunteer searchers.
"On medical advice, the experts believe William is within a 1.5 kilometre radius of the house.
"He may be curled up tight but he should still be in a state where he could respond to calls.
"I emphasise that people must search every blade of grass and return to the command centre satisfied that William is not in the area they have searched."
The search area will concentrate on a 1.5 kilometre radius from the home.
Searchers have been scouring bushland at Kendall for William since he went missing about 10.30am on Friday September 12 from the front yard of his home on Benaroon Drive.
Police from Mid North Coast Local Area Command have been coordinating the search, involving SES volunteers, RFS volunteers, members of the local surf lifesaving club, the police dog squad, mounted police, officers using trail bikes and Polair.
Late on Saturday, police divers arrived to search a number of dams in the vicinity of where the boy was last seen.
The search area has expanded to cover a 10 kilometre area, with in excess of 100 community members registering to search on a voluntary basis.
William has been described as being of Caucasian appearance, with dark hair and hazel eyes. He was last seen wearing a Spiderman suit.