Fire crews are "winning the battle" against the SkyCity inferno that has brought central Auckland to a standstill.

But it has come at a cost, with one firefighter taken to hospital with moderate injuries after being struck by a piece of falling roof.

Fire and Emergency regional manager Ron Devlin said their strategy at containing the fire was working as planned.

"We are without any doubt winning the battle against the fire."

The roof had nearly burned out, about 20 per cent was remaining, and was being contained from spreading through the building by the water they were using.

They expected by 8pm the roof would have fully burned through and smoke begun to dissipate.

Devlin confirmed one firefighter had been taken to hospital earlier this afternoon with a concussion after being struck by a piece of the roof.

"It was unusual, may have been flicked off in the wind. It is not what we want to happen, and the reason we have been very careful with internal operations."

There were about 80 firefighters on the scene currently, down from 130 at the peak.

While the fire had affected the sixth and seventh floors, Devlin said the fifth floor looked in good shape. The roof's membrane was damaged, but structure looked to be intact.

Devlin said he could not say anything about the investigation into the cause of the fire.

"My view is there is no chance of the fire spreading down. I am confident the fire is already contained.

"What we have is a burned structure, still smouldering. Probably about 20 per cent of the roof is still burning, and once that is burned it will have burned itself out."