TVNZ's Channel 1 briefly dropped off the air this evening after a fire alarm sounded in the building, forcing staff to leave the building.

1NEWS Tonight presenter Greg Boyed was reading a late-night news item about the recent car bombing in Afghanistan when he was interrupted around 11pm.

"We have a fire alarm so we'll be back very shortly," he said before cutting to an ad break.

The channel then briefly cut to a broadcast error message, complete with an image of the iconic Goodnight Kiwi.

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Northern fire service shift manager Jaron Phillips said the alarm was set off by a short circuit in the electrical switch room.

Power has been isolated and two fire crews are on the scene, he said. The building has been evacuated.