Truck driver Brett Sullivan was heading back to the yard to knock-off for the day when he came across a car crash.
Sullivan, who works for Hawky Haulage, was driving near Kaukapakapa, northwest of Auckland on Saturday afternoon, when he spotted a car that had rolled.
"It must've happened seconds before I came across the corner and there was a car in front of me that had stopped.
"They were running back towards the car and I could see someone's arm floating around through the windscreen.''
Sullivan, who was a volunteer firefighter for 16 years, said his instincts kicked in and he shot out to see how he could help, as other people called emergency services.
"So I thought: 'I'll just look after whoever's in the car."
Inside the vehicle, he saw a teenage girl who appeared to be dressed in a fast food restaurant worker's uniform.
"She was pretty upset and disoriented.
"She'd been upside down a couple of times and she was still hanging upside down in her seatbelt when I climbed in,'' he said.
"I helped her out of her seatbelt and just tried to keep her calm and as comfy as I could.''
He guessed he was there for at least half an hour, comforting the teen and telling her she was going to be okay.
The Auckland Westpac Rescue Helicopter arrived soon after and she was flown to Auckland City Hospital in a serious condition.
A spokeswoman for the hospital confirmed that an 18-year-old had been brought to the hospital for treatment, but an update on her condition was not possible.
Westpac Rescue Helicopter staff praised the truck driver for staying with the girl until she was free from the wreckage.
Sullivan said he hoped to meet her one day and to let her know he would do it all over again if he had to.
"To be honest, I didn't give it much thought.
"I just jumped in and would hope that anyone would do the same if it was a member of my family who was in trouble,'' he said.
"I just wanted to make sure she was looked after properly.''