Police say it is “vital” they find teenager William Callaghan before he spends a second night in the freezing Victorian bush after vanishing more than 24 hours ago on a walk with his family.

“We are using every resource possible to find him,” Victoria Police Acting Inspector Christine Lalor in an update on the search at 3.30pm on Tuesday.

“It is going to be another cold evening again, hence why it is vital that we try and find William before nightfall. The longer it goes on, the more concerned we have for William,” she said.

Hundreds of police, volunteers and locals are racing against the clock to find 14-year-old William, who is autistic and non-verbal.

With only a few hours of daylight remaining and another freezing night forecast, volunteers came from across the region to help police search and rescue squads at Mount Disappointment, about 60 kilometres north of Melbourne.

As his distraught family and friends waited for news, tradesmen, footballers and other locals left work to join the search amid fears Will would endure another night in the cold and dark.

At least 450 people have been looking for the teenager. But there has been no sighting and no clothing found.

Police say William is energetic and food-focused – they hope he might have walked into a house to help himself to food, or tuck himself into a bed.

“William is quite active so he could have travelled quite a distance – anyone in the area at all, check your house,” Inspector Lalor said.

“William doesn’t have a very good road sense, so anyone who finds William, if he is near a road, [should] be mindful of that.”

She said the search would continue overnight, and as far and wide as needed.

Police say William Callaghan could have travelled quite a distance. Photo: Victoria Police

A mix of four-wheel-drives, people on foot, people riding motor bikes, people on horseback and in the air are all part of the search.

Police were very concerned about how long William would be able to stay outside, with the potential of hypothermia amid “life-threatening” subzero weather in the hills.

“We really don’t want to have someone out here multiple nights,” Search and Rescue Squad Senior Sergeant Greg Paul said.

“There’s no sugar coating it, it’s a very cold night last night. It’s life-threatening cold, there’s no doubt about that, down to zero,”

“When you have someone out lost in the bush, we are fighting against time.

However, Senior Sergeant Paul referred to the extensive five-day long search for teenager Luke Shambrook, who was miraculously found alive.

“A lot of the same searchers are involved with this one here. This is something that happens from time to time. We are very serious about wanting to put our best efforts, put as many searchers out there and we are still very hopeful we can find Will and we are putting our best foot forward as a community.

“The search will continue. We will continue to give it everything we have got overnight and into tomorrow,” he said.

People came from across Victoria to help in the search. Photo: AAP

Will went missing about 2.20pm on Monday after he ran ahead of his family near the summit of Mount Disappointment.

A large-scale search since then has failed to find the teenager, who can only communicate through tapping his chest.

He has been missing for more than 24 hours, and police hold concerns for him as he was wearing only light clothing and had no food or water.

Tuesday’s clear, sunny sky was keeping hopes high of finding him after an air-and-ground search started on Monday afternoon.

SES crews gathered to check maps of the dense forest at Mount Disappointment. Photo: AAP

Senior Sergeant Paul said the area had “very thick bush” from bushfire regrowth, which was hampering the search.

But they were “giving it our best effort” in the steep terrain.

“The only advantage is … there is an opportunity stay a little bit warm if you can snuggle up in the undergrowth,” he said.

The weather bureau expects the temperature on the exposed mountain dropped as low as zero on Monday, and -3 degrees in the valleys. The same weather is forecast for Tuesday night.

Locals on horseback joined the search on Tuesday. Photo: AAP

William was last seen on the south side of the mountain’s summit and could have covered a lot of distance since then, police say.

“William doesn’t verbalise so if anyone finds him the best way to communicate with him is to be patient and calm,” Inspector Lalor said.

“We want to find William as soon as possible.”

-with AAP