The disappearance of an avid hiker in Tasmania six years ago is being investigated by criminal detectives who are re-interviewing witnesses.

Bruce Fairfax, 66, was reported missing on October 14, 2017 from a track in the Duckhole Lake area in the state’s south while on a walk with his wife and their dog.

A fruitless search for the former teacher involving about 60 people, including ex-students and his daughters, was suspended after about a week.

Tasmania Police say Fairfax’s disappearance is now being probed by a criminal investigation branch.

Acting Inspector Michael Foster said police were re-interviewing witnesses and calling for new information from the public.

“As with any missing persons investigation, the case remains open until we find answers,” Insp Foster said on Monday.

“While this is now a criminal investigation, our detectives have not ruled anything out, and will consider all possibilities relating to Bruce’s disappearance.”

Fairfax, who required daily medication for Parkinson’s disease, was described by his wife Louise as a “gentle, patient, generous giant”.

It was reported Fairfax and their dog became separated from Louise when she walked ahead of the pair. The dog was later found.

Duckhole Lake in Tasmania’s southwest national park is a flooded sinkhole, part of an extensive regional cave system.

Police formally suspended their search for Mr Fairfax on October 23, 2017, saying his window for survival had closed.

Police have previously described him as a capable and experienced bushwalker.

– AAP