The plan of attack for the fire raging in at the SkyCity Convention Centre is to let it burn - for now.

The fire started on the 7th floor of the $700m under-construction convention centre in Auckland's CBD about 1pm yesterday.

Since then more than 80 ground-based firefighters using high-powered hoses have been trying to quell flames.

This morning they revealed their battle plan to the Herald.

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A number of fire crews have been on the scene since the fire broke out, with some coming from Hamilton to bolster Auckland numbers. Photo / Dean Purcell
A number of fire crews have been on the scene since the fire broke out, with some coming from Hamilton to bolster Auckland numbers. Photo / Dean Purcell

Fire and Emergency New Zealand assistant area manager Dave Woon said once the roof had burned to a point where there were no hazards - mainly falling objects and material - crews could get into the building and shift their focus.

"We're waiting for the roof to lose its integrity to a point.... where it's safe for us to work.

"It's a trussed construction which makes it particularly difficult."

The fire started on the 7th floor but the 5th had a concrete base so was the best point to fight it from.

"That's where we're going to base our attack from later on," Woon explained to Focus Live.

"At the moment we're defending from the outside, there are a lot of hazards - there's stuff falling down on top of us.

He said hazards included "multiple tonnes of stuff" that could fall on and around crews "at any stage".

The glass facade of the building was also a potential danger.

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"There's a whole lot of balls in the air for us."

Crews were on standby waiting for the indication that the roof had disintegrated to a point where they could get inside.

Photo / Alex Burton
Photo / Alex Burton

He couldn't say how long that might take.

"How long's a piece of string - we don't know," he said.

"In some cases it's better to have quite a bit more fire so we can get that hazard out of the way."

Woon said about 30l per second was being pumped at the blaze, but the wind was blowing it around meaning the work was "arduous".

He said monsoon buckets were not an option.

"That is an uncontrolled way of putting out fire - with the ladder trucks we can direct where we want the water to go, we don't want tonnes of water being dumped in one area."

Woon said while they were letting the roof burn, there was no plan to let the rest of the building go down.

"There's always a chance, but we're not planning for that to happen - that's what we're defending against."

Another fire expert said the blaze was complex due to the fact the convention centre was still under construction.

NZ Professional Firefighters Union president Ian Wright said it was one of the toughest fires crews would ever face.

Photo / Dean Purcell
Photo / Dean Purcell

"This is as serious as it gets. This is a dangerous, dangerous fire.

"The building isn't finished. There may be bits missing in the building, ceiling panels missing, walls missing, other things missing from there that you would expect to be there.

"There's a whole lot of fire happening where you can't see it. It's under the roof, in the floors below and in the voids below, so in the buildings like this there's concealed ducting and concealed walkways and accessways and these can contain fire and make it difficult to extinguish."

The building's construction, in general terms, would have floors and ceilings which go up and across on the same floor and it was crucial - but difficult - for firefighters to get in those areas.

"So what's happening in there with these firefighters, there will be thick black smoke, extreme heat and it will be really punishing. So it's not what you see from the outside with the people on the roof doing what they're doing, a lot of the hard work will be out of sight, in the dark with extreme heat and you're only feeling your way.

"We have thermal imaging cameras which can help us but we rely heavily on our training to move through the buildings like this, extinguishing fire as safely as possible.

"So it's not what you see, it's what you can't see."