Firefighters have asked the public to show understanding as they battle a blaze in Auckland's CBD and resulting in road closures, evacuations and a thick fog of smoke over the area.

The fire started at the under-construction convention centre at about 1pm yesterday.

There are currently 100 firefighters working to put the fire out and while it is managed, it is still not completely under control.

Some people have questioned why it is taking so long to extinguish the blaze and urged Fire and Emergency NZ to do more - or use monsoon buckets to douse the flames.

However they have ruled that out saying it unsafe and not the best way to quell the burning.

They are working to a strategy which includes sacrificing the roof of the complex to remove falling hazards.

Once that has completely collapsed, crews currently on standby will move into the fifth floor and start attacking the flames.

Crews are currently in a defensive phase, trying to stop the fire spreading past the roof.

Once they get inside on foot, they will move to an offensive strategy.

They will be on site for at least another 24 hours.

One the inferno is out investigators will begin piecing together how the fire started.

This morning Fire and Emergency New Zealand assistant area manager Dave Woon said the roof was a major hazard, with "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.

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

The first crews at the scene worked through the night and were sent home to sleep just after 8am when replacement firefighters arrived.

"Our people are tired," Woon said.

"We just ask for some compassion from the public," he said.

"We're working hard, we're doing the best that we can with a particularly arduous fire."

Buildings around the CBD have been evacuated including TVNZ.

Others have closed for the day including the Auckland District Court.

Several main streets remain closed and traffic - gridlocked at peak hour last night - is still crawling through the city at points.

FENZ Auckland region manager Ron Devlin said he was "spectacularly proud" of his crews.

He also acknowledged police and St John.

"While we might be the front of it, there are other men and women doing a great job.".

Fletcher Building CEO Ross Taylor told reporters today that the fire started where blowtorches were being used on bitumen to seal roof joints. The company remained committed to completing the $700m convention centre.

SkyCity boss Graeme Stephens said the fire was "absolutely devastating".

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

"This is as serious as it gets. This is a dangerous, dangerous fire," he told the Herald.

"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."