William Tyrell went missing about 10.30am on Friday from the front yard of a relative's home on Benaroon Drive, Kendall, south of Port Macquarie. He was last seen wearing a Spider-Man suit.

After another day of painstaking searching by hundreds of emergency services workers and volunteers, police admitted on Sunday afternoon that William's chance of survival was steadily decreasing.

"As time passes that window does diminish", Superintendent Paul Fehon said.

A family friend, speaking on behalf of William's parents, said they were desperate to find the child.

"William is a much-loved and cherished little boy," said the friend, who wanted to be known only by her first name Nicole.

"The family are desperate to have him home, and if anyone knows anything please contact police."

The search had been widened to a 10-kilometre area, however police believe the boy is within a 1.5 kilometre radius of the house. Superintendent Fehon said the search would continue overnight and on Monday, with extra resources due to arrive from Sydney.

Earlier on Sunday, Inspector Kim Fehon told searchers it was a "critical day" for William, who is likely to be in poor health after going without food and probably water.

"I have briefed all the search teams that they are looking for a small boy who is likely to be curled up and may be covered and have found somewhere to hide," she said.

"We need to make sure we have covered every blade of grass within that area."

"At this time we are hoping he is still in a position to respond to being called out to."

Inspector Fehon said the search had not found any evidence pointing to William's whereabouts and police were also looking into the possibility the boy was abducted.

"We are looking into other scenarios and that's why we've got investigators working on that today," she said.

Police from Mid North Coast Local Area Command have been co-ordinating 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.

SES units are involved from as far afield as the Hunter and the lower mid north coast towns of Pacific Palms and Forster.

One initiative involves a mounted section of people on people on horseback including the Camden Haven Pony Club, as well as others with dogs and some riding trail bikes.

A search on Saturday by police divers of a number of dams in the vicinity of where William was last seen failed to find any sign of him.

"The family wants to extend a heartfelt thank-you to all of the police, the SES, every volunteer and every community member that has come together over the last few days to help with the search," Nicole said.

William was left alone with his sister for no more than five minutes in the front yard of the house, which sits at the end of a quiet street, next to bushland.

Kristen Dorward was one of those who said the anguish of William's family had her searching on Friday and Saturday.

"We will keep going," Ms Dorward said.

"I have a two-and-a-half-year-old and a five-year-old. This situation is unimaginable."

William has been described as being of Caucasian appearance, with dark hair and hazel eyes.