A tiny balloon is inserted to push his bladder and rectum out of harm's way during the procedure.
The 63-year-old and his wife, Sally, who built the luxury B&B Glenora Estate on Waiheke Island and ran a realty business in Devonport, came across the treatment after looking into alternatives to surgery.
They sold their Devonport house to help pay for the trip and the proton therapy, which is also available in the United States and Europe, but at a much higher cost.
"I would sell my soul if it was going to get my husband the right treatment," Mrs Holland said.
While at the hospital Mr Holland is being filmed by the South Korea Medical Tourism Corporation, which promotes medical treatments in the country for international patients.
The Ministry of Health says New Zealand has no plans to offer the treatment.
Proton therapy
Proton therapy uses proton particles from a cyclotron instead of high-energy x-rays from a linear accelerator, which is the conventional form of external beam radiotherapy.