Australian police Thursday expressed growing concern for the fate of a young Belgian hiker missing in remote northwest Tasmania, describing the frigid conditions in the region as "not survivable".

Celine Cremer was last seen on June 17, in a small town in the island's scenic northwest.

Her white Honda CRV was found parked at a trailhead in the Tarkine rainforest on Tuesday but is believed to have been there for at least nine days.

Police said she left the car "equipped for a light walk".

Police and family have become increasingly worried, with a search involving ground crews, drones and helicopters hampered by bad weather.

Nighttime temperatures touched freezing this week, with rain and snow cloaking the region.

"Weather conditions have been challenging over the past few days," Inspector Anthea Maingay said Thursday.

"Unfortunately, we have received expert medical advice that indicates the recent conditions are not survivable for the length of time that Celine is thought to have been exposed in the wilderness."

"The search effort is continuing but we are dealing with extremely dense bushland and the challenging weather conditions are expected to worsen over the coming days."