The horse died in 2013 and the SPCA had several run-ins with Power over the last decade.
Power was banned from owning or exercising authority over animals for 10 years and ordered to pay the SPCA $4710.05 for its expenses. She was also ordered to surrender any animals she owned.
But Power took her case to the High Court, appealing the conviction and sentence. She was allowed to keep animals until the appeal hearing.
She also applied for her sentence to be suspended pending the appeal outcome.
In a newly-released High Court decision, Justice Pamela Andrews said Power had five previous convictions relating to animal welfare and control of animals.
The appeal centred on whether Power ill-treated Pip, whether this ill-treatment was reckless, and whether Pip died as a result of reckless ill-treatment.
Power felt there'd been a miscarriage of justice and contended that Judge Dawson made findings of fact for which there was no evidence, or insufficient evidence.
At the High Court in Auckland, Justice Andrews said she was not persuaded Judge Dawson erred in concluding Power had ill-treated Pip.
But she said Judge Dawson did not expressly outline and address "separate subjective and objective elements of the test of recklessness."
Justice Andrews said the process by which Judge Dawson determined Power's guilt was unclear.
More importantly, the High Court judge said there was no doubt Power loved and cared for all her animals.
"She fully believes that she takes good care of them," Justice Andrews added.
She said since Power believed she was taking good care of Pip, it could not be be inferred she knew the horse was at risk of dying due to her treatment of him.
"It follows that the Judge could not have found that Ms Power consciously appreciated a risk of Pip's dying, or indeed a risk that Pip might be suffering pain or distress unnecessarily."
The conviction for recklessly ill-treating the animal causing its death was thrown out.
But the High Court judge said Judge Dawson was not wrong to conclude malnutrition resulting from periodontal disease was a substantial cause of Pip's death.
Justice Andrews said it was right to consider an alternative charge against Power.
"That conclusion does not preclude old age also being a contributing cause of death. However, I am satisfied that the evidence before the Judge was a sufficient basis for finding that ill-treatment was a substantial and operative cause of death."
Justice Andrews convicted Power on the alternative Animal Welfare Act offence of ill-treating an animal.
Because of this modified conviction, Justice Andrews said Power's sentence should be reviewed.
The matter was sent back to the District Court for sentencing on the new charge.
Justice Andrews said the SPCA was not seeking forfeiture of Power's animals.
The Ministry of Justice said Power would appear in Auckland District Court for the re-hearing on October 5.
In 2003, the SPCA seized dozens of animals from Anne Power's rural property after concerns about animal welfare.
Power, who at one point reportedly called herself Dame Anne, was convicted in 1998 for not giving dozens of cows and horses proper and sufficient food.
She was then banned from owning dogs for five years after being convicted of owning two dogs that attacked a person.
