The blazing SkyCity Convention Centre could potentially collapse if the fire burns long enough, causing the steel structure to buckle and fold, experts say.

An Auckland-based structural engineer, who did not want to be named, said the building had the potential to collapse if the fire continued to burn.

The fire has been blazing for more than 16 hours and on Tuesday evening, Fire and Emergency assistant area commander Stephen Sosich said it was getting bigger.

The engineer said while it was difficult for engineers to comment without knowing the exact structure of the building, if any steel building burnt long enough the steel work could deform and parts of it could collapse.

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The massive complex is made up to 8500 tonnes of steel framing supplied by Culham Engineering used to reinforce the concrete slabs poured onsite.

"In the general sense of steel buildings if the fire burns for a while they don't collapse but if it keeps going on and on and on it does. It sounds terrible," he said.

"It's just the nature of fires. Obviously with steel and concrete the structure itself is not on fire, it would just respond to whatever is burning there."

Fire and Emergency NZ have confirmed the fire is wedged between two concrete layers in the roof, making it difficult for firefighters to access.

The engineer said even without the building collapsing, due to the extent of the fire the smoke damage to the building would be massive.

"You could expect the effect on the rest of the building, especially if it's almost finished, the fit-out and everything to be horrific from a smoke damage point of view. It sounds really bad."

While the building should have a dual supply sprinkler system, it was difficult to know whether it would have been operational at this stage.

"One of the unfortunate things is a building still under construction is vulnerable because all of the fire protection systems are not necessarily in place.

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"It will be a very long and interesting investigation."

Another expert, who would not be named, told the Herald there were a number of different composites that could have been used on the roof and it would be the roof and not the membrane which was on fire.

He confirmed steel could collapse if it was exposed to heat long enough.