Multiple people are dead and several have been injured after a large fire broke out at a Wellington hostel this morning.

Fire and Emergency has confirmed there are multiple fatalities and say they are expecting that number to rise.

Crews were alerted at 12.25am to the fire at Loafers Lodge on Adelaide Road in Newtown.

Police say their initial assessment is that the number of deaths is fewer than 10, but they can't be any more specific at this point.

The building is unsafe, and until they can enter and reconcile numbers of people who are safe they are not in a position to determine how many people are dead or unaccounted for.

They ask people to come forward if they, or someone they know, had been staying at the property.

Fire and Emergency deputy national commander Brendan Nally says people need to understand that given the scale of the tragedy, authorities have to be certain before they provide information on the number of deaths.

"There are parts of the building that are significantly damaged and firefighters' safety is very important so we are searching those parts very carefully and deliberately but unfortunately we believe the number of fatalities will rise throughout the day."

Firefighters now have a full register of those who were living in the building. Some people had fled the scene in the aftermath of the fire so it's difficult to track everyone down.

That's contributing to uncertainty about the death toll, says Nally.

Some residents did not have a clear way out.

"They had difficulty getting out and unfortunately and tragically quite a few haven't got out."

Parts of the roof, the mezzanine floor and other areas are giving rise to concern for the structural integrity.

"That's driving our approach."

Photo: RNZ / Samuel Rillstone.

No sprinkler system

Nally confirmed there was no sprinkler system in the building, but there was a dry riser system which means that firefighters could pump water into them and use them for firefighting.

Investigations are already underway but it's apparent a resident had set an alarm off, it did not happen through a smoke alarm.

"We have to work very carefully though our investigative processes and make sure for the benefit of all New Zealand, the who building industry, we understand exactly what happened here, and more inportantly, what we can do to stop it happening again."

There are some fire escapes on the exterior of the building.

The first crews did a thorough search early on "the best they could" but it's necessary now to take even more care.

Nally says he's incredibly proud of firefighters' efforts this morning, including a crew who rescued people from the roof using an aerial truck.

Several people had been plucked to safety from an area directly above the fire.

"Those people were going to perish except for the intervention of our team. The young officer that led that did an outstanding job. Multiple people are walking around because of it."

Earlier, 52 people were taken out of the building and at least five were rescued from the roof, but others remain unaccounted for.

Five people were taken to hospital, two in a serious condition.

A further 15 people were treated by ambulance staff at the scene, but did not require or did not want hospital treatment.

It's a "worst nightmare" situation, Pyatt says, and a once-in-a-decade fire in the city.

"Our thoughts at this time are with the families of those who have perished, and with our crews who valiantly rescued those and attempted to rescue those that they couldn't".

Crews were working inside the building to extinguish the fire and a drone was also being used in the effort.

Wellington district assistant fire commander Michael Dombroski says the fire quickly spread.

"Several occupants were rescued by firefighters and removed from the building."

Kainga Ora says it's not aware of any of its clients living in Loafer's Lodge when the building caught fire early this morning.

It's working with partners and those supporting the residents to see how it could help.

Power has been restored to the immediate vicinity of the building.

According to the Loafers Lodge website, the building has 92 rooms, as well as lounges and kitchens for guests to use, and offers short-term and long-term accommodation. There are three floors of accommodation above the ground floor.

Ambulances remained at the scene at 5am though there were fewer on site than at the peak of the fire. Twenty fire trucks were operating at the site, Fire and Emergency said.

Adelaide Road between John Street and the Basin Reserve is closed.

Wellington City Council says buses and other traffic between the central city and southern suburbs is likely to be diverted for several hours.

Fire trucks, ambulances and police were at the scene, and part of Adelaide Road was closed, as crews battled the fire broke in the multi-storey building. Photo: RNZ / Denise Garland.

-RNZ.