Workers fled for their lives as a massive inferno enveloped the under-construction Sky City International Convention Centre.

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.

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.n Then all of a sudden it was flames and really dark smoke."

Farman said a man in an orange construction vest was standing on top of the building when the fire started.

"He was standing at the back for about four minutes but the he just ran when the flames started to spread".

The giant fire is burning from the top of the SkyCity Convention Centre, which is still under construction between Hobson and Nelson streets.

Thick black smoke billowing from the building can be seen across the city.

Hundreds of people phoned 111 as flames could be seen leaping from the building top at about 1.15pm.

Reporter Khalia Strong is at the scene.

"The smoke clouds are going above the cranes which are above the building."

"It's black smoke and it's very thick. The wind is pushing it towards the Sky Tower."

Black smoke from a fire at the Sky City Convention Centre construction site in central Auckland. Photo / Supplied

NZME employee Alastair Merrett said he saw plumes of smoke coming from the building.

"Then the smoke started to grow. Flames then appeared and spread throughout the whole top of the building."

An office worker in Kingsland said the fire was "epic".

"It's black, huge black smoke. It's big."

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."