The 39-year-old man appears to have spent up to four freezing nights in rugged Watagans bushland before he was found alive on Tuesday suffering from hypothermia.

Police are still investigating the reason for why the 39-year-old had travelled to the area, west of Cooranbong, and not returned.

The man was reported missing by a family member on Sunday afternoon.

Around the same time, a camper in the Watagans National Park raised the alarm after seeing a man two days earlier, on Friday, getting out of a car and entering the bush.

It is understood a camper saw the man park his car at a remote location on Friday and walked into the bush.

When the camper returned on Sunday and discovered the car still parked in the same position, he called police

NSW Police and Emergency Services scoured the area and found the missing man just before 11am Tuesday morning about 200 metres from Murrays Run Road.

He was stabilised by paramedics before being winched out by the Hunter Westpac rescue helicopter and flown to John Hunter Hospital at 2.20pm Tuesday afternoon.

The man remains in hospital in a critical condition.

He is being treated for hypothermia and minor cuts and abrasions, NSW Police said.