Inspector Kerry Watson said those injured are in their "mid-teens".
One person has critical injuries, while another is moderately injured. Two other people are being treated for smoke inhalation.
Neighbours described hearing a loud explosion and the smashing of glass.
Chad Tuara lives nearby, and was picking up his daughter when he heard a loud bang and ran over to the house.
"Two girls were rolling out from a window. My partner and I ran over to help out, there was no one else around," he told Fairfax.
"I was putting water on them, the guy could see and called police and emergency services.
"I was putting water on her, her clothes had pretty much melted on to her skin. She was screaming, 'my arm, my arm'. I think she broke her arm.
"They had come out of the bottom door, straight through some glass. Another girl fell down the stairs and hit the ground."
Watson said they are yet to determine the cause of the fire, but it is not thought to be suspicious.
A Fire Service spokesman said the fire was over a small area, but materials stuck to the skin of the people who were injured.
One of the occupants said they went upstairs and lit a cigarette, and a lighter was found in an upstairs bedroom.
Tamea Vaisima lives on Lappington Rd and told the Herald she didn't hear the explosion but her grandchildren saw emergency services at the address on their way home from school.
"I can see a lot of fire trucks around and police, it's pretty scary."
A witness posted on Facebook they were on Lappington Rd when they saw one female burnt from her hips down and another person who was also "really burnt". "Jesus be with the family on Lappington," they wrote.
A local resident described seeing a woman burning "from the hips down" and another person badly burned at the address. "Believing gods there right not (sic)."
A photographer at the scene said there was no visible fire damage to the outside of the two-storey property.
There was a small cordon surrounding the property and a reserve opposite the property. "It was in the second floor in a bedroom. There was a couple of windows open but that's it."
A Housing New Zealand spokesman confirmed it was a Housing New Zealand property and that an area manager was on the way to the address.
