Kelly Savage, 27, had been teaching English in Japan for two years when a pre-existing mental health condition worsened.
His Wellington-based family say he became manic after stopping his medication because of the side effects.
He was admitted to Yamato Hospital under a compulsory order and restrained on a bed in a secure ward for 10 days.
A nurse found him in cardiac arrest in mid-May and he died seven days later.
His death certificate lists the cause of death as hypoxic ischemic encephalopathy caused by cardiopulmonary arrest.
But his family say the cause of the cardiac arrest is inconclusive and it has been suggested to them that deep vein thrombosis may have been involved because of the long period of restraint.
They say researchers in Japan have reported that 30 days' restraint is common there.
The family have tried unsuccessfully to get medical records from the hospital, which also declined to allow an investigation into the cause of death by an outside party.
The hospital has also declined to apologise, with the chief doctor denying responsibility.
Kelly's older brother, Pat Savage, who lives in Japan with his wife and young children, said he was driven to tears of anger and frustration at a meeting with hospital chiefs yesterday.
He said they told him nurses would have removed Kelly's waist, wrist and leg restraints for short periods on occasions, to wash him or allow him to eat, but would not say for how long or give him the nurses' records.
"I kind of broke down and [was] crying and angry at them because I've been trying to get these records for almost two months now, and they know that I wanted it, and they just screwed us over by, you know, trying to drag the process out as long as possible."
He said whether the restraints were removed "for a few minutes" to allow Kelly, who was sedated, to be bathed was immaterial; he did not need to be physically restrained for so long.
"He does need to be in a hospital - I was glad he was in a hospital - but he didn't need to be restrained to the bed in my opinion."
Dr Savage said it was bad to treat anyone that way, but particularly his younger brother, who was helping Japanese students.
"The fact that Kelly was here ... to try to help international relations, trying to teach Japanese children English, and then he's just dying in this kind of outrageous circumstances that would never happen in New Zealand should be an embarrassment to Japan."
Kelly's mother, Martha Savage, a professor of geophysics at Victoria University, said what had happened was shocking.
"It just seems medieval to me. I mean we were just shocked when we first found out and it seems like it's something from a movie back in the Middle Ages. It doesn't seem like a modern society would be doing this [restraint]."
She said the only thing the hospital staff did for Kelly while he was restrained was put compression stockings on him.
She said restraint was needed to prevent people from hurting themselves or others, but it was usually for a short period of time.
"No more than a few hours and only if they're actually actively trying to resist and trying to go after other people, but Kelly had already stopped resisting at that point and they still put him in the restraint."
Prof Savage said the family did not want anything other than to prevent it happening to anyone else.
"We don't want to sue anybody, we don't want money. We just want other people to not go through this terrible situation again."
She urged the New Zealand government to push Japan to change its practices.
This story was first published on the RNZ website.