A two-storey commercial laundromat erupted into flames in Auckland this morning.

Fire Service northern shift manager Megan Ruru said a passerby raised the alarm at 3.50am after flames were seen shooting out of a laundromat in Copsey Pl in Avondale.

She said more than 50 firefighters fought the blaze.

The fire, which had taken hold at the back of the building, has now been contained.

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Ruru said no one was reported in the building at the time and the fire posed no danger to surrounding buildings.

A fire investigator was now working out how the blaze started.