Dams have been drained and paddocks patrolled, but there is still no sign of the missing three-year-old.

Police said the search will continue on Monday, with extra State Emergency Services resources arriving this morning.

William was last seen at the Benaroon Drive, Kendall, home he was visiting about 10.30am on Friday.

The poet's village and its surrounds have been inundated with searchers in the three days since the child's disappearance.

Hundreds of them used the meeting point at Kendall Showground on Saturday and Sunday to register with emergency services, before combing shoulder-to-shoulder through surrounding scrub.

Superintendent Paul Fehon said on Sunday afternoon that, despite painstaking searching by hundreds of emergency services workers and volunteers, the little boy'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."

Inspector Kim Fehon said police were looking into other scenarios, "and that's why we've got investigators working on that today."

Police from Mid North Coast Local Area Command are coordinating the search, which has included SES volunteers, RFS volunteers, members of the local surf lifesaving club, the police dog squad, mounted police, officers using trail bikes and PolAir.

Kristen Dorward was one of those who said the anguish of William's family had her searching on the weekend. "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 is described as being of Caucasian appearance, with dark hair and hazel eyes.

The search continues.