The blaze, which started at the Princess Street building about 5pm, can be seen from as far away as the North Shore. When the first firefighters arrived, the 50m x 30m warehouse and hardware retail store was totally alight.
Northern fire communications shift manager Jaron Phillips said the fire had now spread to three adjacent buildings.
Twenty appliances from all over Auckland were attending the fire.
Reports of downed power lines, power lines on fire and transformers on fire were also being received by the Fire Service for the surrounding Onehunga area.
Additional fire crews are dealing with these incidents, Mr Phillips said.
Large amounts of smoke was also drifting over high tension power lines.
Mr Phillips said so far there had been no reports of injuries.
Chief fire officer at the scene, Larry Cocker, told Campbell Live the store was filled with tents, gas burners, camping gear and plastic tables.
He said it was a "volatile" and fast burning blaze.
He said the firefighters first had to deal with the ferocity of the fire.
"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.
Police had closed the South Western motorway, but have now reopened it to traffic.
However, the block surrounding Mitre 10 has remained cordoned off to pedestrians and the road opposite the building was closed to traffic.
Senior Sergeant Anthony Edwards of northern police communications said it was expected that emergency services would be at the site for some time and traffic diversions may be in place for hours.
