An electrical fault is being blamed for the cause of a fire which destroyed the Warehouse Stationery building in Rotorua.

The fire is believed to have started in a fluorescent light and Central Lakes specialist fire investigator Stuart Booten says the fire is not suspicious.

The Warehouse Stationary was engulfed in fire by the time crews arrived.

He says it's most likely the fire started in a part from a fluorescent light known as a capacitor, a device of one or more pairs of conductors separated by insulators to store an electric charge.

Stuart says the fire would have developed over night when the lights were switched off and it would have been a long, slow build-up to the full fire on Thursday morning.