The fire that burned for three days last year at the SkyCity International Convention Centre has been found to be accidental.
The roof of the under-construction centre was significantly damaged in the blaze that erupted in October last year, and took 10 days to fully extinguish.
Fire and Emergency New Zealand have released a report into its findings on the cause of the fire.
Fire and Emergency Te Hiku regional manager Ron Devlin said it was inadvertently caused when the top layers of a waterproofing membrane - or cap sheet - were being laid on the roof.
"Our investigators concluded the fire started after the cardboard inner of a cap sheet roll was momentarily exposed to a flame from a worker's gas torch and began to smoulder," Devlin said.
"After smouldering unnoticed inside the roll, an intense fire developed that burnt through the top layer of the cap sheet, and then spread across the level 7 roof membrane."
Fire and Emergency NZ found this to be consistent with tests and a scenario recreation they had conducted.
Strong winds had also contributed to significant spread of fire across the roof, investigators say.
The findings bring an end to speculation last year that a worker had left a blowtorch on and went for lunch, with investigators adding that there were no inexperienced contractors working in the area at the time.
Devlin said crews spent a huge number of hours working on the fire, with nearly 30 appliances and about 150 firefighters tackling it at its peak.
More than eight million litres of water used to tackle the huge blaze ended up in the carpark, where it rose to 1.5 metres and submerged about 100 cars.
During the fire, Auckland's CBD came to a standstill with roads closed around the convention centre, most central businesses shut, and people asked to stay away from the billowing smoke.
Fletcher Construction chief executive Peter Reidy said their own investigators had come to a similar conclusion to FENZ.
"We are pleased that the origin and cause of the fire has been determined and that FENZ confirms that the fire was accidental," Reidy said in a statement.
"It should be noted that this [waterproofing membrane] is an extremely common product with a common application process across the industry."
Fletcher Construction relied on subcontractors for specialist work when dealing with a major project, he said.
Reidy said the subcontractors working on the roof that day were experienced and "employed by one of the two largest specialist roofing membrane installers in Auckland and the firm was approved by the product manufacturer."
"We were pleased with our safety processes on the day including the safe evacuation of 550 workers in less than 30 minutes from the site."
A review has been undertaken of Fletcher Construction's torch-on roofing membrane processes and frontline workers have had facilitated workshops, Reidy said.
SkyCity Entertainment Group chief executive Graeme Stephens said he welcomed the report.
"We thank FENZ for completing and releasing its report into the cause and origin of the NZICC fire and we are now reviewing these findings," Stephens said in a statement.
"We remain committed to the NZICC project and we look forward to works recommencing safely on the site."