Police, who have not ruled out foul play or misadventure in Mr Nichols' disappearance, have renewed calls for anyone with information to come forward.
An investigation by the coroner found Mr Nichols, who served in the Australian army from 1960-65, had died, likely around September 18, 1989.
The father-of-one was involved with the Vietnam Veterans Association and had four days earlier rendezvoused with several other men to build a shack at Lake Sorell.
Mr Nichols stayed behind when the group returned to Hobart on September 17, saying he would be there when they came back the next weekend.
"We don't have clear answers in regard to Ian's disappearance and the circumstances of what occurred," Tasmania Police Detective Senior Sergeant Nick Bowden said.
"It could have been misadventure, it could have been as a result of foul play or suspicious circumstances. Our investigation remains open to all situations and scenarios."
Mr Nichols had a history of mental health issues and said he wanted to stay at the shack to "get off the drugs and booze", the coroner noted.
There was an unconfirmed report he was at a pub at Bothwell, about 35km south of Lake Sorell, on September 19.
Tasmania Police have appealed for information on missing Vietnam War veteran Ian David Nichols. (Rob Blakers/AAP PHOTOS)
Mr Nichols was formally reported missing to police on September 25 - a search of the shack construction site was undertaken four days later.
In his 2012 report, coroner Glenn Hay said in hindsight it was not readily apparent why a coordinated land and water search of the Lake Sorell area wasn't conducted.
Police had information Mr Nichols was in a poor mental state and was last seen in a remote area, he said.
In 2022, police revisited Mr Nichols' last-known movements and interviewed people who either saw or spoke to him in the days before his disappearance.
"Understandably, Ian's disappearance was traumatic for his family and loved ones," Det Sgt Bowden said.
"Families always deserve answers to what happened to their loved ones, and we owe it to them to find those answers."
Mr Hay couldn't rule out the possibility Mr Nichols took his own life or had an accident or suffered a medical episode in bushland.
"While police investigations did not identify the involvement of any other person in (his disappearance) ... I cannot exclude that possibility," he said.
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