Hundreds of police volunteers and family are continuing a desperate search in dense bushland for an autistic teenager who spent the night in potentially “life-threatening” conditions.

William Callaghan, 14, was at Mount Disappointment – 60 kilometres north of Melbourne – with his family when he became separated about 2.20pm on Monday as they were climbing the mountain.

William is non-verbal and there are serious concerns for his safety.

Victoria Police’s helicopter, search and rescue squad and dog squad have joined local uniform members in 4WDs, on bikes and horseback, SES crews, dozens of volunteers and William’s family in the search.

Senior Sergeant Greg Paul of the search and rescue squad described overnight conditions in the area as “life-threatening”.

“There’s no sugar-coating it,” he said.

“It was a very cold night last night, life-threatening cold, no doubt about that, down to zero or close to zero.”

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Searchers are racing to make the most of sunny conditions to find the missing boy before another cold night closes in.

A ground and air search for William started on Monday afternoon and continued overnight. The desperate hunt continued at first light on Tuesday.

William was not wearing appropriate clothing to endure freezing conditions, with temperatures in the area dropping to -2 degrees in Whittlesea, about 10 kilometres away.

Police said William was walking with his family to the summit when he ran ahead of the group and vanished.

He was wearing blue track pants and a blue hooded jumper. He is non-verbal and communicates by tapping his chest.

Police hope William might have found shelter in a building or house.

A steady stream of volunteers has arrived in the area to help with the search.

Senior Sergeant Paul said teams were giving it their “best effort” to find William, who might have sought shelter in the thick undergrowth.

“It is very difficult countryside, steep terrain, hard going. From Mount Disappointment, it’s a large tract of state forest and there’s a lot of area to get lost,” he said.

“From past experience, we know that it can be very difficult and take a long time to find somebody if they’re well and truly lost in this sort of terrain

Senior Sergeant Paul said the situation was “very distressing” for the boy’s family, adding “we don’t want this to turn bad”.

“We’re pulling out everything to try to find this young fellow. It’s very distressing, obviously, for him, but for the family and the whole community.

“We want to find Will as soon as possible.

“So we really appreciate all the people coming out today, last night, searching all night and today, with the sun coming up and the bit of warmth that that brings in, we’re feeling like, you know, it could be a positive day. We hope,” he said.

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He was last seen on the south side of the mountain’s 800-metre summit, and could have covered a lot of distance since he became separated from his family, authorities say.

He is described as energetic and food-focused, leading police to believe there’s a chance he might have walked into a house to help himself to food.

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Acting Inspector Christine Lalor said on Tuesday that anyone who saw William should approach him calmly.

“He will normally tap his chest to communicate, so if anyone finds him, if they can call triple-zero straightaway but just approach him in a calm manner,” she said.

“We can only hope he’s found cover somewhere at least to stay warm.”

James Behan and three colleagues were on their way to work when they decided to take the day off and help the search, the ABC reported.

“There was a mate up here overnight helping out as well, so we thought we would take the day off and do what we can,” he said.

“Obviously it’s cold but we were going to work and then changed our minds and came up here, it’s the least we can do.”