The search for missing three-year-old Anthony "AJ" Elfalak has continued this afternoon, so far returning no trace despite exhaustive efforts.

A major search involving more than 130 emergency volunteers is currently underway in New South Wales' Hunter Valley, with police saying that they are investigating foul play as line of enquiry.

"We're looking into everything," a spokesman for NSW Police told 9News.

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Anthony, who lives with autism, was last seen at a home on a rural property on Yengo Drive, Putty, south of Singleton, about 11.45am yesterday.

He has not been seen since, sparking a widespread search throughout the night and into Saturday afternoon.

Searchers are currently wading through "very difficult terrain", which is expected to become more treacherous as afternoon rainfall sweeps across the area.

"Obviously, the terrain is very difficult. We're in a rural setting, there are various levels of elevations," Superintendent Tracy Chapman said.

"We have been searching some of the dams and waterways, every minute is important so our priority is to find AJ as quick as we can."

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Superintendent Chapman said dams in the area had been emptied and police divers deployed, but have so far returned no trace.

She said that officers are currently assessing the requirement for an Amber Alert.

There are grave concerns for the boy's wellbeing as he lives with autism and is non-verbal.

The rural town of Putty is located about two hours north-west of Sydney and is surrounded by bushland.

Police are using trail bikes, police helicopters and divers, and the Dog Unit to urgently try and track down the boy.

Emergency crews are scouring the remote area with the State Emergency Service (SES), Rural Fire Service.

A command post has been set up down the road from the property where about 30 cars of concerned friends have gathered.

NSW Police issued a statement insisting assistance from the public was currently not required, reminding people of the COVID-19 restrictions in place.

"There are sufficient emergency services on scene and public assistance is not required at this time," NSW Police said in a statement.

"This area is deemed regional NSW and travel restrictions under the Public Health Order apply."

The boy is described as Mediterranean/Middle Eastern in appearance and has short dark hair. He is believed to be wearing a grey jumper and pants, and sneakers.

Anyone with information about this incident is urged to contact Crime Stoppers on 1800 333 000.