Neighbours made attempts to enter a Dunedin home engulfed in flames in hope of saving the occupant, but were beaten back by the intensity of the blaze.
A body has been recovered from the scorched remnants of the Nutsford St home in the suburb of Brockville, after fire tore through the flat about midnight on Friday.
Senior Sergeant Brian Benn said a fire investigator and CIB staff were at the scene on Saturday, but it was too early to comment on how the blaze started.
"It's an unknown cause,'' he said.
A body was found at the flat, but police had not yet completed formal identification and would not release any further details until this had taken place.
A Fire and Emergency New Zealand spokesman said firefighters were called to the blaze at 12.02am and the first unit arrived about six minutes later to find the home well alight.
The Housing New Zealand flat was one of four in the multiplex.
There was smoke damage to some of the other flats, neighbours said. It could be seen where the fire had burned through the roof cavity into an adjoining home.
Neighbours told the Otago Daily Times of the terrifying speed and intensity of the fire.
"I have never seen anything like that,'' Ngatuaine Taana said.
"I was scared. I could feel the heat and the windows were popping out. They were just bursting out - it was crazy.''
Ms Taana believed the fire had started at the rear of the property.
"There were flames everywhere,'' she said. "There was heaps of smoke. It was just blazing smoke.''
She got out of her own home, fearing the fire would spread.
A neighbour who lived in the multiplex said he mistook the noises from the fire for someone cutting up wood.
"Then I heard glass smashing,'' the man, who did not wish to be identified, said. "I thought it was someone smashing my window panes.''
He went outside to check and was confronted by the blaze.
"The heat was so intense. It was very fast. We heard things banging and exploding. Pop, pop. Bang, bang. It was almost like Guy Fawkes night.''
Other neighbours had attempted to rescue the flat's occupant.
"Four or five people tried to get in but the whole thing was so involved, so they just didn't have a show,'' he said.
"It was too fast and too volatile. They were beaten off.''
Ms Taana said the occupant of the flat was a middle-aged man, who required a wheelchair.
"He lived alone,'' she said. "I have been here eight years. He's been here longer than me."
Snr Sgt Benn said police hoped to be in a position to provide further details of the blaze later on Saturday.
A St John spokesman said two ambulances were sent to the scene, but the person had died by the time paramedics arrived.