He has said he was taking his youngest son Lance Junior, who has Duchenne Muscular Dystrophy, to the movies in Rotorua on March 13.
"As we turned up there was a car parked [in the disability park]," he said in a Facebook video.
"Dozens and dozens of times in the past five years, we've experienced this awkward situation where we go to find a car park for our boy who has a powered wheelchair, and we find that it's occupied.
"The most concerning time is when it is occupied by people that don't have a permit ...
"I took Lance into the movie, and out of frustration at this happening, I found a brick and I put that brick through this person's window. I'm regretful of that," he said.
O'Sullivan had since apologised to the vehicle's owner and was making financial reparations.
"I regret the fact I let my frustrations on that evening boil over. I took the law into my own hands."
O'Sullivan was named New Zealander of the Year in 2014 and is the founder of the Moko Foundation.
His career has focused on child health - he created New Zealand's first digital health programme, iMoko, which delivers healthcare to children across the country.
Late last year O'Sullivan told the Rotorua Daily Post he had strong links to Rotorua.
"I started my career in Rotorua, my family and children whakapapa to Rotorua and it helped form me to be the person I am today as a clinician."
O'Sullivan is also renowned for his work to improve health outcomes for whānau in the Far North.
