Firefighters are battling a fire in the Auckland CBD where the roof of a building is ablaze.

Fire and Emergency New Zealand (Fenz) shift manager Carren Larking said an automatic alarm was triggered at a building on the corner of Nelson St and Victoria St West shortly before 4pm on Wednesday.

"Upon inspection, smoke was found on the third floor of the building. When they got to the third floor they saw fire on the roof."

Fenz crews activated another alarm, requesting more manpower and water to fight the fire.

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Three appliances are in attendance, with another four on the way.

There are no reports of injuries. St John and police have been notified.

More to come.