More than 80 firefighters battled the ferocious fire at the building on Princess Street for over three hours before finally dousing the flames.
Before they were able to, however, the blaze spread to three adjacent buildings and across to power transformers and power lines.
All those fires were now out, northern fire communications shift manager Scott Osmond told NZPA.
At the height of the fire police closed down the South Western motorway, which caused chaos for peak traffic and travellers bound for the airport.
A number of other roads were also closed and people in nearby buildings were evacuated.
Twenty appliances from all over Auckland attended the fire and were now being replaced by fresh trucks.
"We'll be there for many hours yet," Mr Osmond said.
So far there had been no reports of injuries.
Chief fire officer at the scene, Larry Cocker, told Campbell Live the firefighters were met by a "volatile" and fast burning blaze.
"It's an old building, the fire was rapidly spreading through the complex, the roof was exploding off and it was underneath high tension wires.
"We had a separate fire at a transformer at another street, so we had a number of things on the go all at once."
He said residents and workers in the area started to "self-evacuate" when things began to explode in the building.
The man who discovered the fire, shopper Brent Morrissey, told Newstalk ZB he was alerted when he smelled plastic burning.
"It just seemed to get strong very quickly and I turned around and the wall at the side of the building was on fire."
He said the second he saw flames, he started yelling at staff to leave the store.
