No trace of Scott Murphy has been found despite four days of searching, Tasmania Police Inspector Steve Jones said on Tuesday.
The 43-year-old's family contacted police on Saturday morning when Mr Murphy failed to return from property he owns in the South Nietta area.
He left on foot, and has no known possessions with him.
Inspector Jones said police commenced searching in the area on Saturday afternoon, but despite significant effort they had had little result so far.
Inspector Jones said there was one possible lead in the Upper Castra area, but further investigation did not lead to Mr Murphy.
"Obviously as the days progress without the successful finding of Scott we do hold grave concerns, but we will continue searching," Inspector Jones said.
But on Sunday morning, several members of Mr Murphy's family and his friends joined the search on quadbikes, ATVs and horseback.
A radius of 17km from land in South Nietta had been determined as the search area.
"This is where we believe Scott may have been heading," Inspector Jones said of where Tasmania Police is operating their forward command post from a State Emergency Service bus parked on cleared forestry land.
"He's got some property very close by, he owns private property very near here."
On Tuesday the command post was being battered by intense rains and cold winds, and Inspector Jones said a coming cold front would likely drop temperatures in the area below zero over night.
He said the "testing" weather and the "difficult terrain" made for a dangerous search, and for that reason members of the public were asked to not join the search unofficially, as doing so might unnecessarily endanger other lives.
"We certainly don't want anyone else who is not correctly trained and appropriately equipped to take our resources away from trying to look for Scott."
He said the search operation will continue with advice from a specialist search and rescue medical professional and Mr Murphy's family.
He thanked the SES volunteers and Mr Murphy's friends and family for their efforts during the search so far.
Scott Murphy is 175cm tall, of medium build and balding with blue eyes. Anyone who may have seen him is asked to call Tasmania Police 131 444.