A fire ripped through a four-storey hostel in New Zealand's capital in the early hours of Tuesday, killing at least six people as others fled for their lives, with some plucked from the roof.

Towering flames and thick smoke could be seen pouring from the top-floor windows of the Loafers Lodge hostel in central Wellington during the night, as 80 firefighters and 20 trucks battled the blaze.

Firefighters found six bodies inside the charred building, but a roof collapse on the top floor prevented them from searching all of the 92-room hostel, emergency services said.

Police said the final toll could not be confirmed until they had entered the building for a "methodical" examination once it was made safe, probably on Wednesday.

Six people were taken to hospital, one in serious condition, the city's ambulance service said. An additional 15 people were treated at the scene.

More than 50 people were rescued.

Firefighters used a ladder truck to save people trapped on the roof, said Fire and Emergency deputy national commander Brendan Nally.

"They plucked quite a few people off the top of the roof from an area directly above the fire," Nally told local media.

"There was no other way. Those people were going to perish, except for the intervention of our team. Multiple people are walking around because of it."

There were no sprinklers in the hostel, Nally said.

The fire was being treated as "suspicious", Fire and Emergency New Zealand region manager Bruce Stubbs told reporters.

But police stressed it would remain "unexplained" until their investigators could examine the scene.

- 'I had to jump' -

One of the hostel residents, Tala Sili, said smoke had rushed in under his door before he decided to jump to a roof two floors below.

"I was on the top floor and I couldn't go through the hallway because there was just too much smoke, so I jumped out the window," he told Radio New Zealand.

"It was just scary, it was really scary, but I knew I had to jump out the window or just burn inside the building."

Hemi Lewis, 56, still smelled of smoke as he recounted his escape from the third floor of the hostel, which had been his home for four months.

"I opened my door and there was just smoke everywhere. There were flames coming from the roof. I couldn't breathe. So I hit the floor and crawled the three flights down to the bottom," he told reporters at an evacuation centre.

"I have no idea what to do now, I've lost everything. It's all gone. I'm in the same clothes."

Some residents said the building's smoke alarm went off so often that they doubted it was an emergency when it rang during the night. Others said they heard nothing.

About 90 people were thought to have been in the building when the blaze began, fire services said.

- 'A dark day' -

Prime Minister Chris Hipkins described the fire as "an absolute tragedy".

Hipkins praised the "incredible effort" of firefighters to extinguish the blaze and evacuate people.

"I acknowledge the victims and their families -- an absolutely tragic set of circumstances," he said, promising a "thorough review" of the disaster.

"There will be an opportunity to test whether this building was fully compliant with all of the rules that it needed to be compliant with but obviously the focus at the moment is supporting our firefighters," Hipkins said.

The New Zealand leader said many shift workers were staying in the hostel, making it difficult to know how many people were in the building.

Wellington Mayor Tory Whanau said the hostel was home to a mix of long and short-term residents, including some on lower incomes or staying in the country on a "transitional" basis.

"For our Wellington community, it does feel like a dark day," she told a television interviewer.

The Loafers Lodge advertises itself as a "convenient and affordable" option offering laundry and kitchen facilities as well as security, with a lock on each floor.

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