The cells are used to retrieve contraband from people suspected to have concealed it internally and usually contain no water.
State custodial inspector Richard Connock uncovered instances of people being held for up to 10 days with no window to the outside world and only one hour of exercise per day.
There was no intercom in some of the cells, meaning inmates had no means of calling for help in an emergency.
Camera monitoring and physical checks by staff occurred but were not always reliable, Mr Connock said.
"These conditions are harsher than those imposed on people who are segregated for disciplinary reasons," he said.
"Notably, contraband was very seldom found in the dry cell placements we reviewed."
The conditions breached a United Nations' Nelson Mandela rule stating "all prisoners shall be treated with the respect due to their inherent dignity and value as human beings", he added.
One prisoner said isolation in a "dry cell" for 72 hours with only a paper and pencil negatively impacted his mental health.
The prisoner said he had to "scream" at staff to have the lights turned off at night and no contraband was found.
Mr Connock said it was deeply concerning inmates were unable to call for help, 33 years after a royal commission into Indigenous deaths in custody raised the issue.
He said he was aware of "dry cells" being used in other Australian jurisdictions and called for them to be phased out.
The report validated concerns raised by a worker in 2022 that inmates were in "inhumane" conditions.
Tasmania's prison service in February said some procedures had changed, including around lighting, available food and ensuring a mattress was provided during the day and night.
However, inspections in May revealed the changes had not been implemented.
The state justice department conceded the correct balance between prison security and ensuring inmates were treated with dignity had not always been met.
The report made more than a dozen recommendations around "dry cell" protocols, including intercoms and ensuring lights were turned off at night.
Minister for Corrections and Rehabilitation Madeleine Ogilvie said all recommendations were being adopted.
She said soon-to-be-rolled-out body scanners would provide an opportunity to review "dry cell" protocols or stop the process altogether.
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