Paul Temple was relaxing on a couch in his front yard, enjoying the morning sun and a cigarette, when a boy ran over to him yelling, "Help, my brother's on fire".
A few metres away a 14-year-old boy was caught in a blaze in a shipping container on a neighbouring property.
The boy, his younger brother and another boy had been fooling around with lawnmower fuel when one of them lit a cigarette, sending the 14-year-old and the surrounding furniture up in flames.
Two of the boys escaped the container but the 14-year-old was trapped.
Mr Temple, 59, and originally from Fiji, could hear cracking noises and saw smoke and flames coming from the container at the Maeroa house in suburban Hamilton.
He ran over to help but immediately thought the worst.
"The boy was calling out to help him and I tried but the container was too hot and I couldn't get in ... There were flames everywhere, they were blazing out of it," said Mr Temple.
"Somehow he managed to get out and fell to the ground so I ran to the bathroom and came back and sprinkled water on him."
Mr Temple said that by the time paramedics arrived about 10 minutes later, he had tried to comfort the boy the best he could but he was a mess.
The boy suffered burns to 70 per cent of his body; his 11-year-old brother suffered minor to moderate injuries.
"He had burns on his front, on his back, his feet and some of his hair was gone ... It was horrible. He couldn't talk or move," said Mr Temple.
Fire safety officer Ross Fleet said firefighters arrived at the scene to find the container well ablaze shortly after 11am. He said there were no suspicious circumstances and the incident appeared to be nothing more than "a very serious accident with an element of foolhardiness".
He said the others told him there was a "woof" sound minutes after the boy lit the cigarette. "The cigarette was discarded then the furnishings and his clothing caught fire in rapid fashion," said Mr Fleet. "We need to be vigilant especially in the use of fuel and the casual use of lighting a cigarette."
Waikato District Health Board spokeswoman Mary Anne Gill said the boy's condition was serious and he had been airlifted to the national burns unit at Middlemore Hospital.
Waikato police communications manager Andrew McAlley said Mr Temple was instrumental in saving the boy's life.
He said Hamilton police were assessing an appropriate way to acknowledge his actions.