The brother of murdered Ashburton teen Kirsty Bentley has spoken of his shame at doubting his father's innocence in a new appeal for information on the 23-year-old mystery.

Both the 15-year-old's brother John Bentley and father Sid Bentley were treated as suspects by police after Kirsty's body was found 18 days after she disappeared while walking the family dog on New Year's Eve 1998.

But after eight years working on the sprawling cold case file, Canterbury CIB's Detective Inspector Greg Murton said last month he didn't believe John or Sid Bentley were involved in Kirsty's death.

His "stranger-type abduction" theory instead involved a loner killer who lived locally, either smoked or grew cannabis, and knew the rugged Rakaia Gorge area where Bentley's body was eventually found.

A $100,000 reward for "material information or evidence" that brings about a prosecution, and possible immunity from prosecution for any accomplice, has been offered.

Detective inspector Greg Murton believes a stranger or strangers were involved in the death of Kirsty Bentley. Photo / Jason Oxenham

Being treated as a suspect had never made sense and left him feeling "helpless", John Bentley told Newshub Nation this morning.

"You know you didn't have anything to do with it and yet they're still pushing and pushing … you're just relying on them finding evidence to prove who the killer was, to prove your own innocence."

It'd been particularly hard on his father, who remained living in Ashburton until his death in 2015 aged 64.

"[People] would gossip. And I'm ashamed to say that at some point I began to wonder why his story had changed, began to doubt him, and it just upsets me that he died not knowing that he didn't have to go through that.

"He died thinking people thought he was a liar, and that was wrong."

Eighteen months after the murder, Sid Bentley changed his story of where he was the day Kirsty disappeared, putting him potentially closer to home than in Christchurch - where he had been that day.

His memory changed after hitting his head, he told police, but he later said he'd been confused and reverted to his original statement.

John Bentley was the last person known to have seen Kirsty after she left the family home to walk dog Abby, later found tied to a tree at Ashburton River near where Kirsty's underwear and boxer shorts were found in a nearby bush - a scene Murton now believes was stage by the killer to throw police off the scent.

Jill and Sid Bentley, pictured with Kirsty's dog Abby in 1999. Photo / NZME

Both father and son have vehemently denied any involvement in the teen's death.

He didn't like to think about the detective's new theory too much, because it had previously been thought his sister died quickly and "probably didn't know what was happening", John Bentley told Newshub Nation.

"The new conclusion opens up some scary possibilities that aren't good to think about."

Ashburton teen Kirsty Bentley disappeared on New Year's Eve 1998. Photo / File

However, he urged anyone with information relating to when Kirsty disappeared to contact police, especially given a new profile of a potential killer had been released.

"The crucial bit of evidence we need, someone might not be aware of what they've got … I'd rather people flood the helpline with everything they know and let the police sort out what's important", he told Newshub Nation.

"I have to be Kirsty's advocate, Mum has to be Kirsty's advocate …we need to remind people Kirsty existed."

- NZ Herald