Police are still combing through the information they have received in the hunt for Kirsty Bentley's killer.

About 80 tips – regarding people, vehicles, and events – had been sent in since a $100,000 reward was posted in late July.

The reward was for any information or evidence leading to the identification and conviction of the person or people responsible for the Ashburton 15-year-old's death, which remains unsolved.

Detective Inspector Greg Murton said he was about three-quarters of the way through the nominations.

Some were random sightings and many required a lot of digging to contact all relevant parties.

Nothing obvious had emerged yet.

Kirsty (right) was last seen alive on the afternoon of December 31, 1998, as she left home to walk her labrador cross, Abby.

Abby was found the next day tied to a tree near the Ashburton River, but Kirsty's body was not discovered until 17 days later in the Rakaia Gorge.

Initially, the investigation focused on Kirsty's father, Sid, and brother, John, but both denied any involvement.

Sid died of cancer in 2015, while John now lives in the United Kingdom.

Murton has looked over the cold case for eight years. His primary theory was she was abducted and killed by a stranger, who lived locally, either smoked or grew cannabis and knew the Rakaia Gorge area.

He did not believe Sid or John were involved in her death.

The reward will remain in place for another four months.

Immunity from prosecution may be considered for any accomplice, not being the main offender.

Anyone with information is asked to phone police via 105 and reference Operation Kirsty, or Crime Stoppers anonymously on 0800 555 111.

- Tina Grumball, starnews.co.nz