The explosion which tore through a Northland house, badly burning a 19-year-old woman, blew out the windows some distance and could easily have hurt bystanders, the local fire chief says.
The woman suffered burns to about 70 per cent of her body and was taken to the burns unit at Auckland's Middlemore Hospital following the explosion at the house in Ruakaka, south of Whangarei, on Monday evening.
She remained in a critical condition on Tuesday morning.
She was the only person in the house, believed to be a 100sq m two-bedroom home, at the time.
Nearby houses were rocked by the blast.
The woman may have opened the door when the explosion happened, says Ruakaka fire chief Darrell Trigg, but fire safety investigators on Tuesday were trying to find out exactly what happened.
"It's very rare that you actually get an explosion of that sort - 100 per cent fully involved from one end of the house to the other in a split second," he told NZ Newswire.
Police say it may have been a gas explosion.
Mr Trigg agreed that was possible - there was a gas heater in the house. It also may have been a smoke or backdraft explosion, he said.
The windows were blown out quite some distance and it was lucky no one else was nearby, he said.
The Ruakaka fire service was training not far away and was able to get there quickly. An ambulance crew member also lived not far down the road and was also there in good time.
When firefighters arrived the woman was being looked after by a neighbour.
The single-storey house was so badly damaged it will have to be knocked down, he said.