The fire hadn't threatened any other properties, a spokesperson said.
Herald readers have sent through photos of the smoke plumes going across the sky at Wiri.
One resident was driving past and says black smoke is spilling from National Scrap Metal, on Hobill Ave, across the motorway and over towards Papatoetoe.
"I went past about 20 minutes ago and there's thick, black smoke spilling across the motorway towards Papatoetoe.
"But it's not the first time it's happened."
Scrapping cars involved not only fuels and oil, but sparks from cutting landing in the wrong place proved dangerous.
"They do a lot of scrapping of cars in there so you know, there's all the fuels and oils and one spark and it's all over.
"They do a lot of gas cutting, it's metal on metal which creates sparks and you got fuel around, she's all over."
The witness said police had since blocked off several roads.