William Callaghan, 14, became separated from his family on Mount Disappointment, north of Melbourne, without food, water or warm clothing.
Temperatures plunged to below zero as more than 100 searchers scoured the challenging terrain throughout the night.
Senior Sergeant Greg Paul from the search and rescue squad, said there was “no sugar coating” the perilous nature of the conditions, for William and the massive search party of more than 300 people.
“This is life-threatening cold,” Sen. Sgt Paul said.
“This is very thick bush,” Sen. Sgt. Paul explained, adding the large search zone was now covered in bushfire regrowth.
“This hampers searchers and it is very difficult to see someone on the ground from the air.”
The positive side of that, Sen. Sgt Paul said, was that the undergrowth could offer William some shelter and insulation from the bitter cold.
Police and volunteers have been searching the large tract of state forest, and adjoining farmland, on foot, horseback, motorcycle and from the air.
William was wearing blue tracksuit pants and a hoodie when he went missing.
He is described as very energetic and food-focused, leading police to believe there’s a chance he may have walked into a house to help himself to food.
Residents in the Mount Disappointment vicinity have been asked to check their bedrooms and outhouses.
Police said he would be hungry and searching for food.
William can only communicate with hand signals and by tapping his chest.
“From past experience we know it can take a long time to find somebody in this terrain,” Sen. Sgt Paul said.
“We are giving it our best shot — we are pulling out everything to find this young guy.”
