As the search for missing three-year-old NSW boy William Tyrell nears the end of the third day authorities and volunteers are beginning to worry about his fate.

“As time goes on, obviously the survival diminishes,” Superintendent Paul Fehon told Network Ten.

“After three days looking for a young person, I think it’s fair to say we would have liked to have located something at this stage.”

William went missing from the front yard of the home of a relative, believed to be his grandmother, at Kendall, on the mid-north coast, on Friday morning.

The area where police, SES, members of the Rural Fire Service, local surf lifesaving club and several hundred community volunteers are looking for William was expanded on Sunday.

Authorities have been combing bushland for the past few days and on Saturday evening police divers began searching surrounding dams for the boy, who was last seen wearing a Spider-Man costume.

The search will continue throughout Sunday night, following what local police inspector Kim Fehon described as a “critical day,” given William had likely gone without food and water for several days.

“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 told Fairfax.

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

Jenny Atkins, a senior instructor at the Camden Haven Pony Club who’s joined the search, said the volunteer’s mood was “pretty down” and “worried”.

“We’ve had riders going out helping on horse back looking through the thick scrub,” she told AAP.

“It’s rough country, they come back with leeches,” she said.

Since the search began the close-knit community has rallied around William’s grandmother, Ms Atkins said.

“Everyone’s feeling for her,” she said.

“Our community does support each other, we’re very rural.”

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

A police spokeswoman said there weren’t any plans to wind the search back.