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," said Murton.
"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.
"Our ultimate goal is to provide answers and a sense of closure for Kirsty's family.
Murton said any information would be assessed closely and the Commissioner of Police will determine the amount of the reward and will, if necessary, apportion payment where there is more than one claimant.
The reward will remain on offer for a six-month period.
Anyone with information is asked to contact Police via 105 and reference Operation Kirsty.
Alternatively, you can call Crime Stoppers anonymously on 0800 555 111.
In 2016 a coroner ruled that Kirsty died from a massive blow to her head on the day she went missing.
The finding came 18 years after she died.
