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Then he saw a terrified little boy peering out a window and heard his desperate screams for help and fists banging against the glass. The 20-year-old could not walk away.
Mr Harwood, from Athenree in Bay of Plenty, joined the NZ Fire Service at 16, and is now living in Canada with his girlfriend, Stephanie Saxton.
On Saturday, the couple were driving to the resort town of Whistler when Mr Harwood's rescue skills were called on - just 14 months after he saved a woman in heavy flooding at Waihi Beach in April last year.
The drama in Canada unfolded when he and Ms Saxton spotted a plume of smoke and saw a car that had flipped, rolled down a bank and caught on fire.
Mr Harwood slammed on his brakes and ran to the scene.
"The car was completely on fire with five people inside. As a firefighter, it was natural instinct to just get in there," he said yesterday.
"I instantly thought 'it could blow' and so I stood back. But then I heard banging and I looked down and it was a young kid screaming. He was screaming, 'Help, help'. You just cannot walk away from that."
Mr Harwood and a man named Mike Devine, who had also pulled over, tried to get a window open to get to the boy.
"We eventually ended up getting a big rock and smashing the glass. We pulled him out and when we were that close, we could see a little bit more. There were three more people in the back and the driver in the front.
"By that time the fire was getting pretty bad on the bonnet. I was screaming out for a fire extinguisher ... If we didn't get one we would have had to pull out. Someone had one and we used it to buy us some time ... We just needed enough to prevent it for a couple of minutes."
The flames eased slightly, but were still burning. Mr Harwood cut the driver from his seatbelt using a knife tossed to him by a bystander and pulled him free.
They then went to work to rescue the three in the back seat. An elderly man was hunched over another child, and a woman was trapped.
"The fire reignited - that's when it started to get really nerve racking," Mr Harwood said. "It was really intense. The flames were starting to travel up and above us, it was pouring through the dashboard into the car. It was getting really close.
"We believed the old man was dead ... We had to get him out of the way to get the boy. That was more important, to preserve the other life. We got him out and started working on the boy, who had a badly broken arm and leg and stomach lacerations ... As I pulled him out, the fire was getting massive.
"It was almost too much. I reached in and grabbed the last person, the mum, and yanked her out and just ran. Then the car just exploded. We were just in time."
Another firefighter and a doctor had pulled over, and they started to treat the patients. The elderly man was barely breathing.
Mr Harwood spent about 30 minutes doing CPR but could not revive the man. The rest of the family survived and were taken to hospital.
"We did everything we could. It was sad one life was lost but they are lucky they didn't get burned alive. It could have been so bad."
Yesterday, Mr Harwood looked back at the rescue and recognised the danger he had put himself in.
"The focus is the now. It's a situation that had to be dealt with. We've got to do what we've got to do," he said. "It was definitely much bigger than we thought, looking back. Would I do it again? Oh yeah, over and over."
Ms Saxton was proud of her boyfriend, describing his efforts as amazing. "Honestly, this was by far the scariest and most life-threatening thing I have ever seen," she said.
"If it wasn't for Bud and Mike ... that entire family wouldn't be here today. Really proud of Bud and Mike for literally risking their lives to save a family."
Mr Harwood's mother Lorna and father Peter - the chief fire officer at Athenree - were thrilled to hear of his selfless actions.
"He seems to just come up in these situations and he's a boy with no fear and a real heart for helping people," Mrs Harwood said. "He just dives in, even if it's super-dangerous."
Last year, the younger Harwood featured in the Herald after he rescued a woman trapped in her car during heavy flooding near Waihi Beach.
He had been to multiple callouts that night as a result of the bad weather and had just filled his car with petrol when he chanced upon the woman. Her car was partly submerged and Mr Harwood spotted her trying to get out.
He braved knee-deep rushing water to smash a window with his elbow and help the woman climb out.