Emergency services were called to the yard, owned by Global Metal Solutions, after reports a pile of cars at a scrap metal yard was ablaze.
Six crews worked overnight to dampen down any hotspots, he said.
Massive black plumes of smoke could be seen from parts of South Auckland as firefighters worked to control the blaze for several hours.
At one point, the blaze measured about 25m by 15m and locals reported hearing small explosions going off now and again.
An emergency alert was sent via a text and alarm sound to residents' cell phones as a result; warning people to stay indoors and to turn on the air conditioning to recirculate the air, if possible.
But Clarice Ford, who has poor lungs, was only alerted to the potential danger when her neighbour spotted her sitting outside on the deck.
"Not everyone got the text. If [the air] had been poisonous, I would've had breathing problems.
"I was sitting on my deck when my neighbour walked past and told me. The windows [were] open because it's hot."
The 58-year-old had earlier been on a walk with her dog when she saw police officers wearing masks.
She did not know there had been a fire until one of her neighbours asked if she had got "the text".
A second neighbour, Shirley Chapman, also reported not receiving the text. Both women live in Papakura.
"Nobody had said anything. It could've been really dangerous for people who are vulnerable," Ford said.
"We don't all sit with the radio on to hear the news. A lot would not have been aware that it was on, but they would've had their air conditioning going and their windows open."
Fire and Emergency NZ's national communications centres manager Gavin Travers said Kelvin Rd was not within the designated area to which the alert was sent out to.
"We understand people's concerns in this situation and we make every effort to ensure people close by are informed," he said.
"The area which received the Emergency Alert in Papakura was decided according to the size and direction of the smoke plume and on the basis it included the Telcos' cell towers.