Terrifying images have been captured of the massive inferno at the under-construction Sky City International Convention Centre.

Workers fled for their lives, while a worker at the scene said nobody knew what had happened. They suspected the fire had started on the roof of the top floor.

Fire alarms had started and they assumed it was a drill, he said. "Then people said run, run and we knew we needed to get out." He had not seen any smoke inside the building.

A worker who went on a smoko break, leaving a blowtorch unattended, is believed to have sparked a massive inferno.

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There are unconfirmed reports that sprinklers in the $700 million building, which is still under construction, had not yet been installed.

Witnesses have described the moment the fire broke out.

NZME account manager Charli Farman was sitting at her desk when she noticed smoke and told a colleague "I think it's on fire".

"It was a really small amount of thick black smoke. Then all of a sudden it was flames and really dark smoke."

Mayor Phil Goff said the fire appears to be out of control - spreading "massively" in the 15 minutes he and council staff had been watching it.

"The line of flames is going from one side of the building on the West right through to the East."

"The fire engines are on the road but there's no evidence they've got to the roof or what the origin of the fire was."