An explosion in an Otara house that seriously burned two teenage girls was caused when a group of teens lit a cigarette and it ignited the fumes on their clothes.

The two girls remain in Middlemore Hospital but a third person injured in yesterday's blast has been discharged overnight.

A hospital spokeswoman said one of the burns victims remains in a critical but stable condition and the other is stable.

A nearby resident has described the "horrific" aftermath, saying she saw a person lying completely burned on the front lawn, and a teenage girl carried out by a firefighter and stripped down, exposing her burned legs.

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The blast was in the upstairs bedroom of a Housing New Zealand unit, which smashed several windows.

Northern Fire Service spokesman Jaron Phillips said neighbours saw a woman engulfed in flames running from the house.

Police said they wanted to talk to witnesses. Photo / Doug Sherring
Police said they wanted to talk to witnesses. Photo / Doug Sherring

Four of the five people aged between 14 and 20 who were in the house when the fire service arrived shortly after 2.30pm were treated at Middlemore Hospital. Two were treated for smoke inhalation.

Therese Tuitavake had just collected her children from a nearby kindergarten and was heading home when she turned into Lappington Rd and was overtaken by a fire engine, which sped along the footpath.

Tuitavake said an ambulance was already parked outside the two-storey property and she slowed her car.

As she strained to see the fire, she saw a teenage girl being carried out of the house and onto the front lawn.

"The girl had to be stripped. They flipped her over and her whole legs were burned."

Neighbours saw a woman engulfed in flames running from the house. Photo / Doug Sherring
Neighbours saw a woman engulfed in flames running from the house. Photo / Doug Sherring

As the girl was laid on the ground, Tuitavake saw another person lying motionless on the lawn.

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"They were fully burned. Totally burned."

She was unsure whether it was a man or a woman as the hair was also singed.

A group of children walking home from school stopped at the park opposite to watch so saw the people coming out of the house.

"I was just really worried and watching the scene. It was horrific."

Tuitavake kept looking to see a fire burning inside as a large hose was dragged inside, but said there were only a few smashed windows.

A passerby stopped and poured water on the injured girls.

The Fire Service said the fire was only in a small area, but the injuries had been caused by the material melting on to skin.

The blast was in the upstairs bedroom of a Housing New Zealand unit, which smashed several windows. Photo / Doug Sherring
The blast was in the upstairs bedroom of a Housing New Zealand unit, which smashed several windows. Photo / Doug Sherring

Specialist fire investigator Terry Jordan said the explosion was caused by a cigarette being lit, which ignited flammable vapours on their clothes.

"I think they were sniffing it. Either petrol or LPG. Because they've gone upstairs in the house and lit a cigarette or something like that and when they've lit a cigarette it's ignited."

Jordan said a petrol can was outside the back of the property and it appeared they had been "doing their thing" before going upstairs.

"One of them hopped out the window upstairs and one has come running down the stairs and crashed straight through the front window - straight through the glass. I think she was the one worst off."

He said the smoke alarms - one in the bedroom and one in the hallway - appeared to have been removed.

Police said they were speaking to four people but did not know the identity of the fifth person in the house, whom they wanted to talk to along with other witnesses.

Housing New Zealand confirmed it was a state house. Regional manager Karen Hitchcock said the agency was working closely with the relevant services to establish what had happened.

"At this time our thoughts are very much with our tenants and their whanau."

Housing New Zealand properties always had working smoke alarms installed and these were regularly inspected, a statement said.