Fire and Emergency New Zealand investigators have found the cause of the New Zealand International Convention Centre fire in Auckland's city centre on October 22 last year was accidental.

The Convention Centre was under construction at the time of the fire. At its peak nearly 30 appliances and around 150 firefighters battled the major blaze which took ten days to fully extinguish.

Fire and Emergency Te Hiku Region Manager Ron Devlin says the official investigation into the cause and origin of the fire found it was inadvertently caused when the top layers of a waterproofing membrane - or cap sheet - were being laid on the roof of the convention centre.

"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.

"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 seven roof membrane.

"It is worth noting we eliminated anecdotal speculation at the time that an inexperienced contractor caused the fire by leaving a blowtorch going prior to leaving the area for lunch.

"This did not happen. Our investigators found all gas bottles around the origin of the fires were turned off for the lunch break. There were no inexperienced contractors working in the area at the time."

Ron says it's a significant step forward to have the cause and origin report completed.

"The NZ International Convention Centre fire was a complex and dangerous fire and we have been focused on ensuring the investigation into its cause was comprehensive and accurately reflected what occurred on the afternoon of 22 October. An independent peer review agreed with our findings.

"We can now confirm the cause of the fire was accidental and we hope this will assist in the organisations involved to move forward."

Ron says fire crews spent a huge number of hours working on the fire over a 10-day period.

"I want to take this opportunity to thank our people who worked hard on this fire.

"We understand the convention centre fire had a severe impact on Auckland, and we appreciate the patience the city showed while we extinguished it."

You can find the NZICC Fire Investigation Report here.