Convicted double-killer Russell John Tully has been linked to the unsolved 1998 murder of Ashburton schoolgirl Kirsty Bentley.

Tully's serving life imprisonment with minimum non-parole period of 27 years for shooting Leigh Cleveland and Peggy Noble at the Ashburton WINZ office in 2014.

The New Zealand Herald has learned he's a person of interest for detectives investigating the Kirsty Bentley murder.

Detective Inspector Greg Murton, who's leading the cold case investigation, has confirmed the 51-year-old falls into that category.

Kirsty Bentley was 15-years-old when she disappeared walking the family dog on the Ashburton riverbank on New Year's Eve.

Her badly-decomposed body was discovered two weeks later, more than 40 kilometres away in the Rakaia Gorge.