One of the men working on an upper level of the New Zealand International Convention Centre building site at SkyCity says a crane operator working above the roof was among the first to spot the burgeoning fire and raise the alarm.
Craig Merz was part of a team installing a building maintenance unit on an area between levels five and six of the seven-storey building.
He was waiting for the crane to swing the next load their way when instead the operator started sounding his emergency horn.
"We looked up to see what was going on - he pointed behind us and we saw rolls of black smoke. I said 'Ross, we gotta get outta here now'."
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They rushed to the stairs on level 5, at which point Merz realised the general alarm still hadn't gone off.
"We were telling people, you've got to get out, the roof is on fire." But many of the foreign workers didn't appear to understand him.
He began looking for a fire alarm activation point but by the time they had crossed the fifth floor someone had set it off - he estimated between two to three minutes after the crane operator's horn sounding.
Merz - a former firefighter who was involved in a high-profile rescue on the SkyTower in 1997 - said the evacuation of hundreds of workers was well-run.
"It was really good, very efficient - no one was panicking. We were taking the stairs out at a reasonable pace ... just walking out. From inside you couldn't see smoke or fire so people were probably wondering what's going on."
He knew tar was burning as soon as he saw the black plumes of smoke.
"When tar catches fire it burns really fast."
He didn't know who had been atop the roof or whether they had raised an alarm.
From his firefighting days Merz knew emergency workers would be facing a difficult task. The building was huge and there was an "intricate" walkway in the roof made up of 2m-wide catwalks.
"When you're trying to fight a fire and there's smoke around, you've got to know where you're going."