Auckland man Adam MacLean was driving along the Northwestern Motorway when he saw black smoke billowing in Te Atatu.
"I saw fire trucks zooming on the motorway."
Within 20 minutes the black smoke turned white, he said.
Emergency services arrived at Te Atatu's McLeod Rd at 2.45pm today where a large warehouse building was "well-involved in a building fire", a fire spokeswoman said.
A total of 12 trucks attended the scene along with support vehicles to control traffic.
Firefighters were currently dampening down hot spots.
She said there was no one in the building at the time of the fire and there were no reports of injuries.
A fire investigator was currently at the scene.