Kirsty Bentley Photo: supplied
The 15-year-old had been out walking her dog in Ashburton on New Years' Eve 1998, and never came home.
The dog and her underwear were discovered the next day but Bentley's body wasn't found for another 18 days - partially buried 60 kilometres away, near the Rakaia Gorge.
Detective Inspector Greg Murton said someone knew the truth and they should come forward.
"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.
"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."
He said immunity from prosecution may be considered for any accomplices.
The reward would remain on offer for six months.
Anyone with information was asked to contact Police via 105 and reference Operation Kirsty or call Crime Stoppers anonymously on 0800 555 111.