The body of Brendan Percy, then aged 31, was found by trampers in January, 2004, in remote tussock country.
A postmortem found he died of hypothermia.
But that left unanswered questions for his family, which has sought the help of the Sensing Murder programme on TV2 to resolve the mystery.
The story will be featured on next Thursday's episode at 8.30pm.
Oamaru police and the family hope the publicity will encourage someone to come forward.
Mr Percy's "baby sister", Megan Robb, said yesterday the family had always been suspicious of her brother's death.
He had no reason to go into that remote area, would have needed a heavy-duty vehicle to get there, when he only had a bicycle, and he was starting a course at the Pukeuri freezing works.
The family tried to contact him on the Monday he was due to start the course but got no reply.
On the Wednesday, police arrived to tell them his body had been found.
"We always had a gut feeling that it never felt right.
"We never believed he got there on his own," she said.
Asked if the family thought Mr Percy had been murdered or taken to the area and left to die, Mrs Robb said: "We're not really sure.
"It's either one or the other".
"We hope if there is a person involved they come forward, whether it's on or off the books, and talk to police," she said.
The family wished for final resolution - "at least as final as we can get".
"We were very lucky we had the closure of having his body and we could say our farewells.
"We would just like to know what happened.
"We would like to think someone has a heart [to tell us that]," Mrs Robb said.
Detective Sergeant Mike Ryder, of Oamaru, said yesterday there were unanswered questions surrounding Mr Percy's death and he hoped the television programme encouraged people to come forward with new information.
Mr Percy was seen alive with his mountain bike and helmet on Sunday, January 18, 2004, between Kakanui and Oamaru.
His body was found on Wednesday, January 21, 2004.
"We've kept his investigation open as it has never been established how Mr Percy came to be in an area of sub-alpine terrain, why he was there or what happened to his mountain bike and helmet, which were never located," Det Sgt Ryder said.
Mr Percy may have had assistance from others to travel into the Mt Dasher area.
"We know that Mr Percy had a number of associates who used and sold drugs and, if involvement in this or other crime may be preventing these people from coming forward, I encourage them to contact me and discuss their situation," he said.
Police hoped to hear from anyone who had any information regarding the circumstances leading up to Mr Percy's death and what happened to his mountain bike and helmet.
They can contact Oamaru police on (03) 433-1400 or anonymously on Crime Stoppers 0800 555-111.
