Parts of the blaze engulfing SkyCity's Convention Centre are too dangerous for firefighters to battle on the roof and too hard for a cherry picker to reach, officials say.
Fire and Emergency assistant area commander Stephen Sosich said the fire was still not under control, and was getting bigger.
"It is a very difficult fire to put out. The fire is wedged between two concrete layers in the roof. It is too dangerous for crew on the roof, but very hard to reach from our aerial appliances."
• Fire at SkyCity Convention centre: Workers flee giant blaze, black smoke chokes Auckland
• Watch live: SkyCity convention centre fire 'out of control'
• SkyCity convention centre fire: Smell of burning plastic and chemicals as smoke pours from building
• SkyCity fire: Fire and Emergency NZ upscale blaze to 6th alarm
Sosich said they were adequately resourced and it was just a matter of time before they brought the fire under control. Strong winds were also a factor.
Smoke billowing out of the building mixed with melting bitumen layer on the roof was dangerous to people nearby, and fire officials was warning people to stay out of the area.
Industry experts say the fire may have started when a primer, rather than the roofing membrane, ignited.
There is speculation the blaze started when a worker left a blowtorch, which was being used on roofing membrane, unattended during a smoko break.
However, experts have told the Herald the bitumen-based membrane was used on thousands of projects around the country everyday and would be unlikely to catch fire.
The primer, which goes on underneath the membrane, is however flammable and classified as a hazardous substance.
The General Manager of a company that manufactures and imports of roofing membrane, said the membrane is rolled out over the primer and heated up with a naked flame to create an adhesive surface.
While the man, who didn't want to be identified, did not know what happened he said it was possible a person laying the membrane could have left the gas torch on near a primed part of the roof and that was what caught fire, not the actual membrane.
It that was the case he said the worker would have been highly negligent in their work.
"The rule of thumb there is when you are not actually torching the membrane on the roof the torch should be turned off."
He compared it to someone using a BBQ - you turn the gas off when you aren't using it then turn it back on when you come back later on.
Others in the industry said the membrane, which creates a final waterproof and UV resistant surface on the roof, is heat resistant and the torch would need to have been left on it for a long time in order for it to catch fire.
The head of another company said even if the torch was left on by mistake it is very loud and it would be surprising someone didn't hear it