Searchers looking for a missing three-year-old boy on the mid-north coast of New South Wales have re-traced their original steps, to double-check the boy has not been missed close to home.

William Tyrell, dressed in a Spider-Man costume, was playing with his sister outside his grandmother's house at Kendall, south of Port Macquarie, when he went missing on Friday morning.

Superintendent Paul Fehon said the third day of the search saw crews go back over the original search area.

"Our search has gone back to the original location where young William was last seen, and we have retraced approximately one to one-and-a-half square kilometres quite thoroughly," he said.

Police said they would continue to search through the night for the child.

Earlier, Inspector Kim Fehon briefed rescue teams and said they still believed the boy was within a 1.5 kilometre radius of the house.

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

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Inspector Fehon said the search was in a critical phase.

"We're now in the third full day of the search for a three-year-old boy who has had no food and unless he's found water, no water, so it is likely that he has in a poor state of health and we need to find him today," she said.

"We have sought expert medical advice from a doctor who has provided advice on many previous searches with success.

"He has advised us of course the three-year-old is very small and that by today he is likely to be still and tucked up tight."

PolAir remained on standby after searching the area for the past two days while sniffer dogs also covered the area.

Police divers also searched dams in the area.

"All swimming pools were searched on the first day that were visible from the air, so we're confident that we've covered all of those," Inspector Fehon said.

Voices calling out 'William' echo in the bush

Police divers search local dams for the missing boy. ( ABC News )

The community rallied around the Tyrell family with hundreds calling out "William" as they searched dense bush.

Jodie Kelly said she had to come and help.

"I've got two kids and I'd be so devastated to know that my little boy was in this situation," she said.

"We're all out here to help the family. [It's very dense] about a metre and a half high of grass, like he could be anywhere."

Noleen Campbell and her family have been out looking through the bush twice now.

"I'm out here today because I'm a parent and I'd hate to think that my child was out there and I would wish every single person out in the world would come and help me," she said.

"[William would] be terribly frightened and that's why we're all here today ... it's so important to get out here and help."

Ms Campbell said the Tyrell family had their support.

"I think they probably feel a lot of guilt and they shouldn't feel any guilt at all because we are all parents and things happen and people shouldn't be judging them," she said.

Missing child is asthmatic, residents say

Volunteers call out "William" as they search thick bushland at Kendall near where the three year old went missing. ( ABC News )

Local resident Brad Hinder spent the past two nights looking for the child who they believe suffers from asthma.

"We came out just to hope we could hear him crying in the quiet of the night ... but no sound," he said.

"He is most likely in this forest ... it's more thick in there - up to your chest ... a child can easily hide if he's had an asthma attack and gone down or even gone into shock.

"He could have got lost and panicked and gone into shock. It's the only reason you can imagine that he would sit down and be so silent.

"It's treacherous, you couldn't get much worse bush for a young three-year-old."