Firefighters were called to a fire at the Manners St McDonald's.

Flames rose above the golden arches after deep frying turned bad in central Wellington early on Thursday.

The fire, in the Manners St McDonald's fast food restaurant, caused buses between Willis St and Lambton Quay to be diverted for about an hour.

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SHONA GARWOOD/SUPPLIED Flames were seen coming out of the restaurant's roof.

Fire crews from around the Wellington region were called in to battle the blaze, senior station officer Craig Gold said.

When crews arrived, the deep fryer was on fire and flames were coming out of the restaurant's roof.

Firefighters used ladders to pour water down the flue and onto deep fryer.

The restaurant was filled with smoke.

Assistant commander Gareth Hughes said an unattended deep-fryer was most likely the cause but he could not rule out an equipment fault.

Smoke could earlier be seen inside the building.

Power was cut to the kitchen, to avoid a second flare-up.

The situation was causing traffic backups around the city. Bus lanes opened again by about 8am.