Extensive searches of Duckhole Lake failed to uncover any trace of Bruce Fairfax, who was reported missing while on a walk with his wife and dog in the area on October 14, 2017.
Tasmania Police on Monday said the disappearance of Mr Fairfax, an avid hiker and former teacher, was now being probed by a criminal investigation branch.
"Fresh eyes have been put upon it. From that we've started down the investigation we have," Acting Inspector Michael Foster told reporters.
"While it is a criminal investigation, that's just in name. We're looking at every aspect to determine what occurred.
"What we really want to know is the truth of what happened. There are a number of scenarios that are possible."
Insp Foster said police were keeping an open mind to all possibilities, including that Mr Fairfax wasn't at the lake.
About 60 people, including Mr Fairfax's daughters and former students, were involved in the initial lake area search which was called off after about a week.
Insp Foster said further searches were planned in coming weeks.
It was reported at the time Mr Fairfax and their dog became separated from his wife Louise when she walked ahead of the pair. The dog was later found.
Insp Foster said the criminal probe wasn't a result of a tip off but a review of existing information and a need to explore further avenues.
Police will re-interview old witnesses and have urged anyone with information to come forward.
Insp Foster said there were no suspects, but some people had been interviewed.
Mr Fairfax, who required daily medication for Parkinson's disease, was described by Louise as a "gentle, patient, generous giant".
"It's heartbreaking for everyone involved and (the family) want closure as much as we do," Insp Foster said.
Duckhole Lake in Tasmania's southwest national park is a flooded sinkhole, part of an extensive regional cave system.
Police previously described Mr Fairfax as a capable and experienced bushwalker.
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