Inspector Kerrie Brill said in this kind of investigation "anything is a possibility, we don't rule anything out."

Sat, 6.30pm: The temperature is expected to plummet overnight, with meteorologists predicting dire conditions around Kendall.

Weatherzone's Guy Dixon said the good news is conditions will become more stable and sunny on Sunday.

The first night William was lost the mercury only got as low as 13.2 degrees, but tonight it is predicted to fall to as far as about 11 degrees. 

While that temperature is actually higher than the area's September average of 10.2 degrees, Mr Dixon said it was "very chilly conditions for a little one."  

Emergency services will continue to search during the night, and welcome people to join the search tomorrow morning.

Inspector Kim Fehon said there will be a briefing from 8am, but volunteers can arrive at the Kendall Show Ground from 7am.

"Members of the public can turn up at any time to register during the day," Inspector Fehon said.

Superintendent Paul Fehon said police were very grateful for the community's tireless resolve through the first two days of the search.

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.

Sat, 4.13pm: The size of the search for William Tyrell has been increased dramatically, as clear skies are forecast overnight.

Organisers said three square kilometres were searched by 3pm, with another seven square km to be explored later the same day. 

Superintendent Paul Fehon said more than 200 volunteers scoured the area on Saturday, with more resources expected to arrive.

Police divers have been sent to the Mid-North Coast to check nearby dams and waterways. 

Superintendent Paul Fehon said emergency services will continue to search throughout the night, and are buoyed by predicted mild weather.

Weatherzone's meteorologist Ben McBurney said there is only a slim chance of rain, and "most will fall on Comboyne and further west in the ranges."

"We will not give up hope," Superintendent Fehon said.

Police were grateful for the community's commitment and support, and asked people to register at Kendall Showground before joining the search. 

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 went missing during a five minute period about 10.30am on Friday when he was unsupervised while playing with his sister, police said.

Emergency services arrived at the house six minutes after being called.

Sat, 1:50pm: PolAir continue to patrol above Kendall as more and more people arrive to search.

SES controller Paul Burg said he understood why so many continued to turn up, despite police saying search numbers were sufficient. 

But the thick bush being searched can be dangerous for those who are not properly prepared.

"Some people turn up with kids and in a pair of thongs, and they're the ones who end up getting hurt," Mr Burg said.

SES units are involved from as far afield as the Hunter and the lower Mid-North Coast towns of Pacific Palms and Forster, all staffed by personnel who raced to help.

"A lot of them came straight after work, so it's pretty encouraging that people who have stuff on just drop everything to come here," Mr Burg said. 

The SES controller said managing fatigue of volunteers will be essential as the search continues.

Sat, 9.20am: Police revealed this morning they hold grave fears for William Tyrell as he suffers asthma which is triggered by stress.

Inspector Kim Fehon made the announcement to media during this morning's briefing.

She said police have had no sign of the three-year-old since he went missing near a Benaroon Street home about 10.30am on Friday.

The family were "keeping it together," Inspector Fehon said.

"The father's been out all day yesterday, and up at first light this morning searching."

Hundreds had registered with the SES to help volunteers from the Rural Fire Service and Surf Life Saving clubs.

Mounted police, trail bike riders, the local pony club and PolAir will all assist in the second day of the search. 

"We have extensive resources and the search coordinator here who is trained in doing these kinds of searches," Inspector Fehon said.

Search coordinators said the terrain around the house is light bush which becomes heavier scrub outside of that area.

An SES spokesman said a three kilometre square area from the edge of Kendall to the next major house south of Benaroon Drive will be searched. 

"Everything we searched yesterday will be searched again today," he said.

"We’ve got 3 to 4 times the amount of resources we had yesterday including 200 local people.

"We will expand the area once it is completely searched, and we will go out again and again until we find something.

SES volunteers changed shifts at 11pm on Friday night, with the overnight shift going home after 6am on Saturday and replaced by a fresh crew.

"We’re trying to manage fatigue as best we can, but they all want to just get out there and find this little fella.

Organiser said fairly mild and dry conditions overnight would help searchers. 

Sat, 6am: Large numbers are expected to join the search this morning at Kendall Showground, Batar Creek Road.

SES registration is from 7am with the search briefing starting at 8am.

Police asked for people to wear high visibility clothing for wet weather, sturdy footwear and bring plenty of water. 

The search will cover scrub near Benaroon Drive, including parts of the Kendall and Middle Brother state forests.

Weatherzone's Sam Brown said light coastal showers are likely from 8am, and these should taper off after lunch.

How the search unfolded on Friday.