An arsonist may have ignited the hostel blaze that killed at least six people in New Zealand's capital, police has said, as they opened a homicide investigation.

"I can confirm that we are treating the fire as arson," police inspector and acting district commander Dion Bennett told reporters on Wednesday, declining to give the full reasons.

"It is being treated as a homicide investigation."

Police have a list of people they want to speak to, he added, but no one has been arrested so far.

Smoke and flames engulfed the 92-room, four-storey Loafers Lodge in Wellington in the early hours of Tuesday, sending residents fleeing for their lives.

Some survivors crawled through smoke to safety, while others were rescued from the rooftop by firefighters using ladder trucks. Those who died still lie in the charred building.

Firefighters found six bodies inside the hostel, but said they were unable to search everywhere because the roof had partially collapsed on the top floor. Police have said the death toll may rise.

Two hours before the blaze broke out, a couch had caught fire inside the building without being reported to emergency services, police said earlier.

Investigators were looking into possible links between the two incidents, they said.

A police reconnaissance team entered the building on Wednesday for the first time since it was declared safe, to look for evidence and locate the dead.

"The scene examination will be extensive and methodical and we expect it to take some time, likely several days given the large size of the building," Bennett said.

Fire damage inside the building was "extensive", Bennett said, with burnt debris reaching as high as one metre.

Police said 92 people had so far been accounted for, with up to 20 yet to be tracked down.

'This one's real'

One of the survivors, Simon Hanify, said he heard a smoke alarm at the time of the couch fire but ignored it because it often went off without reason.

When the alarm rang again two hours later, however, he left the burning building.

"I wasn't even going to leave my room. But I felt like a cigarette. I thought I'd go outside because I usually share them with other people," Hanify said.

"There was smoke coming down the stairwell on the ceiling and our hallway," he added.

"I've been through fire before so I did a quick lap of our floor, knocking on doors, saying 'this one's real, evacuate'."