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The 1998 murder of the Ashburton schoolgirl remains one of New Zealand's most high-profile cold cases.
Kirsty disappeared on December 31, 1998, while walking the family dog Abby on the banks of the Ashburton River.
The day after Kirsty went missing, the family's dog Abby was found tied to a tree near the river, and the teen's underwear was nearby.
Her body was discovered two weeks later in the Rakaia River gorge, 40km away.
No one has been charged over Kirsty's death, which remains a cold case.
Her father, Sid, was a suspect in his daughter's murder but denied any involvement. He died of cancer in 2015.
Today police announced a reward of $100,000 for "material information or evidence" that leads to the identity and conviction of anyone responsible for the teenager's death.
Kirsty's mother Jill Peachey today welcomed the $100,000 police move, telling the Herald it was a "great reward". She did not wish to comment further.
Detective Inspector Greg Murton said police would also consider a deal for immunity from prosecution for any accomplice who came forward and assisted with the inquiry.
"Police have gone to great lengths to explore all possible scenarios and investigative leads and, even after more than 20 years, we remain committed to holding the offender or offenders to account," Murton said.
"Our ultimate goal is to provide answers and a sense of closure for Kirsty’s family.
"That’s why today we’re offering up to $100,000 for material information or evidence leading to the identify and conviction of the person or people responsible for her death.
"Someone out there knows the truth. We urge them to do the right thing and finally come forward — if not for themselves, then for Kirsty and all those who loved her."
The offer will remain in place for six months.
The police commissioner will determine the amount of the reward and will, if necessary, apportion payment where there is more than one claimant.
Anyone with information should phone police von 105 and reference Operation Kirsty.
Alternatively, call Crime Stoppers anonymously - 0800 555 111.