Firefighters in Dunedin are tackling a fire in a unit, which is threatening properties next door.

Several 111 calls were made just after 9am reporting the fire in a block of four units in Lock St.

Neighbours said a man in his 20s lived in the unit, but he was not home at the time.

A neighbour, who declined to be named, said she looked out the window to see flames and black smoke coming out a front window of the property.

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"He's gone out on his bike. Hate to think what will happen when he gets home."

A woman in a nearby unit is being treated by ambulance staff at the scene.

The cause of the blaze is unknown at this stage but a fire investigator is en route.