Emergency services were called to the building on Victoria St about 10.10am and are currently on the scene.
Fire service shift manager Dallas Ramsay said that there are at least 10 fire trucks and 40 firefighters are on the scene.
"Upon arrival there were reports of a fire. We are still attempting to locate the source," she said.
TVNZ staff have been evacuated from the building, which is undergoing renovations.
About 50 Fletcher Construction workers were evacuated from a neighbouring property in Hobson St.
Speaking outside the TVNZ headquarters, fire service assistant area commander Mike Shaw said they still couldn't find the source of the blaze.
"There's a lot of nooks and crannies - it could be anywhere."
He didn't think the fire was growing.
"Everyone's out, everyone's accounted for. There's definitely smoke and heat.
Senior station officer at Auckland Central Chris Lind said the fire was located in the store room, on the second floor.
"The sprinkler system has done its job but the entire building was heavily smoke-logged.
"The fire is all out and we are happy. All critical areas are free from water damage."
Mr Lind said the officers were using fans to ventilate the building.
