Volkner's brutal slaying — reprised in the 1983 film Utu — made news around a shocked world, especially the claim that Kereopa had removed Volkner's eyes with his fingers and eaten them, earning him the name "Kaiwhatu: the eye-eater".
Accounts at the time reported that the cleric's head was placed on the church pulpit while frenzied warriors who witnessed the slaughter danced and yelled. The next day, Volkner's severed head was taken for smoking before it was carried to other places.
His body, which had been tossed down a long-drop, was eventually laid in a grave dug by local Maori.
The Crown's response was blunt. Colonial troops and Maori allies were sent to Opotiki, seizing thousands of hectares of land and arresting and killing resistors.
It took soldiers five years to catch up with the elusive Kereopa, who had a £1,000 bounty on his head. Eventually he was taken captive in the Ureweras, where he had been protected by Tuhoe people.
At the trial in Napier, a Supreme Court jury took barely 15 minutes to find Kereopa guilty.
Contemporary research presented to the tribunal cast doubt on his guilt, and suggested his trial was anything but fair.
Witnesses who appeared for the Crown were granted immunity from prosecution in return for helping secure a conviction, though it is not clear the court was aware of the arrangement.
Kereopa, for his part, was unable to get anyone to testify in his defence because the Crown refused to help them travel to Napier for the trial.
Te Rangikaheke Bidois, lead negotiator for Ngati Rangiwewehi, said the pardon was a bittersweet outcome. For descendant's of Kereopa, the fact his name had been cleared was immensely important, she said.
But the whanau might want the pardon to go further and remove the stigma that had burdened his family for generations. Mrs Bidois said there was a history of suicide among male descendants of the Arawa chief. "It's hard to undo the shame which his whanau has felt," she said.
Kereopa is the second important figure to be pardoned over Volkner's killing. Mokomoko, a Whakatohea chief, was one of several Maori tried and executed soon after the murder. He was pardoned in 1992 by Governor-General Dame Catherine Tizard, and legislation restoring his character, mana and reputation was passed last December.
Mrs Bidois said Kereopa's whanau might want the same acknowledgement, but it was for them to decide. Besides the pardon, the iwi received $6 million in reparations, an apology for Treaty breaches, transfer of a forest and the return of several culturally valuable springs around Lake Rotorua. Two decades of work had gone into the claim, Mrs Bidois said, and while they had hoped for more, the iwi wanted to build on their assets.
For author and film-maker Peter Wells, the Crown's pardon came too late to be included in his new book about Volkner and Kereopa, Journey to a Hanging.
His account — a blend of history, imagination and biography — charts the collision course of the immigrant minister with the influential Kereopa, a disciple of the Pai Mairie ("Good and Peaceful") faith, an indigenous religion with Old Testament roots, and the subsequent hanging of Volkner's accused killer.
Wells recounts that the tragedy had an inevitability once Volkner, against all advice, returned to Opotiki from Auckland, where he had been visiting his wife Emma. The churchman was viewed with deep suspicion because he had come to be seen as a Government spy.
"He was heading to his doom," said Wells, and the Whakatohea people, who built Volkner's church, could not alter the fatal outcome.
Kereopa, for his part, arrived in the Bay of Plenty with a heavy heart and possibly revenge on his mind. The year before, his wife and two daughters died near Te Awamutu after British troops burned down a whare where missionaries told the family they would be safe. The next day, in another Waikato seige, Kereopa's sister was killed.
Volkner had sent Governor George Grey a plan of the pa where the family burned to death.
Says Wells: "There was going to be a victim." The only doubt was who would be chosen - Volkner or Thomas Grace, a Taupo missionary who fled his parish because of war.
Wells' book includes a portrait of the prisoner Kereopa taken in Napier prison by the town's photographer, Samuel Carnell. Just days from the gallows, Kereopa stares mournfully from the image, a cloak pulled up beneath his chin and his moko traced with a marker and added after the shot was taken. The garment was placed to conceal a serious neck wound which the incarcerated chief had inflicted with a concealed razor in a vain attempt to defeat Pakeha justice.
On his last night on Earth, after the influential colonist William Colenso had tried but failed to get clemency for Kereopa, Catholic nun Mother Mary Aubert kept the prisoner company "in a fight for his soul".
Wells believes both Volkner and Kereopa faced their fate with courage.
