Dozens of workers have fled for their lives and one person is missing after a massive fire broke out at the Sky City casino convention centre in downtown Auckland.

The blaze, which started about 11.15am (AEDT) on Tuesday, reportedly ignited when a worker went on a break and left a blowtorch unattended on top of the seven-storey building.

Within minutes, it had turned into an inferno, sending thick, black smoke into the air above New Zealand’s biggest city.

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The $700 million convention centre, which is under construction, is near the centre of the Auckland CBD. The New Zealand Herald says there are unconfirmed reports that sprinklers had not yet been installed in the building.

Workers could be seen fleeing the blaze, which prompted hundreds of calls to emergency services as flames were seen leaping from the roof of the centre.

❗️UPDATE 3: Fire in Auckland CBD

Fire fighting crews wearing breathing apparatus are attending the fire.

20 appliances and 16 specialist and support vehicles are attending the incident including a fire investigator and operations support.

— Fire and Emergency New Zealand (@FireEmergencyNZ) October 22, 2019

Fire and Emergency New Zealand assistant area commander Dave Woon told a media conference at 1.30pm (AEDT) that firefighters were trying to account for one person.

“We are not on top of the fire, but with the resources that have just arrived we should be able to make progress,” he said.

Fire authorities sent more than 100 firefighters and nearly 40 vehicles to the scene, including aerial appliances from Hamilton (100 kilometres south of Auckland).

#skycity on fire pic.twitter.com/0KNIfyMdld

— Laurence Metry (@LaurenceMetry) October 22, 2019

Mr Woon said the huge amount of firefighters on the scene represented “Auckland at its capacity at the moment”.

“If we were to have another fire of this significance then no, we don’t have enough resources,” he said.

Two layers of insulation, apparently straw and 10-centimetres thick, were ablaze, he said.

“We are effectively working from the inside,” Mr Woon said.

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Several city streets have been closed and Aucklanders have been urged to avoid the CBD or expect major delays.

The fight to control the blaze has been complicated by up to 70km/h winds across the city on Tuesday. New Zealand’s weather bureau said the winds are forecast to strengthen later in the day.

-with AAP