The mistake meant Mr A received a triple dose of the maximum radiation, of 93.4 Gray.
A report into the incident by the Health and Disability Commissioner said it wasn't clear whether the mistake happened as a result of human or technical error.
But after the mistake was discovered, ARO started its own investigation that found a stressful working environment was the root cause of the incident.
It recorded that staff had higher than usual levels of overtime, and that senior planner staff levels were inconsistent.
After the mistakes in his treatment, a wound developed on Mr A's back. He needed continuous dressings on it, which his wife usually provided.
He was also unable to drive, or walk up stairs.
Mr A died at home in 2017, but the Commissioner's report notes that it didn't investigate his cause of death, only the quality of care he received.
The Commissioner has found ARO breached the Code of Health and Disability Services Consumers' Rights, by not providing services with reasonable care and skill.
ARO has since made a number of changes to avoid a repeat incident, including a pre-treatment check of all parameters on the first day of a patient's treatment.
An electronic programme for second checking has also been introduced, to eliminate human error in treatment.
The Commissioner has recommended ARO send a written apology to Mr A's family for breaching the code.
It's also recommended anonymised details of the incident are shared with other radiation oncology departments throughout New Zealand to ensure there are proper policies in place to stop it happening elsewhere.
