Robert Charles Mansell died in August 2015 while fishing with his best friend Joshua Kennedy.

Despite a lengthy inquest into the 42-year-old's death, and more than five years passing, Coroner Olivia McTaggart was unable to determine how he was killed.

Releasing her findings in October this year, Ms McTaggart flagged failures in the initial police investigation, which she suggested could have looked into the possibility Mr Mansell was murdered.

"In hindsight, a more critical approach to the suspicious features of this case was required at an early stage and may have resulted in more evidence being obtained which could have clarified the circumstances of Mr Mansell's disappearance," she wrote.

IN OTHER NEWS:

The 2018 inquest found inconsistencies in the story of Mr Kennedy, who was the last person to see Mr Mansell alive.

The coroner put the inquest on hold, asking police to re-investigate, and Mr Kennedy was again interviewed.

But more than two years later, the case remains unsolved, and Mr Mansell's body still has not been found.

After the findings were reviewed, acting Deputy Commissioner Jonathan Higgins said "Tasmania Police has considered the comments by the coroner and will take them into account when conducting future investigations".

Anyone with information about Mr Mansell's death should contact police on 131 444.