Firefighters are still at the scene of a dramatic high-rise blaze that ripped through an office tower on Auckland's North Shore.

Fire service northern shift manager Megan Ruru said one engine was still at the BDO tower on Como St watching for fresh flare-ups inside the damaged building.

At the height of the blaze more than 20 engines and 80 firefighters were battling the explosive fifth floor inferno, which broke out just after 9pm.

The blaze ripped a hole in the side of the building. Photo / Supplied via Brandon Rhodes
The blaze ripped a hole in the side of the building. Photo / Supplied via Brandon Rhodes

A fire safety officer is investigating to determine the cause, she said.

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Lumino The Dentists' Takapuna practice in Como St is open today - and is offering free checkups for the firefighters who extinguished the blaze.

Lumino chief executive Andy Tapper said the fire was contained to the upper floors and there was no damage to the practice, which opened its doors at the usual time today.

"The Takapuna practice was unaffected by the fire, which was mainly due to the great efforts of the local fire service."

The Lumino team on site during the blaze were impressed with how the many firefighters handled the situation.

In acknowledgement of their efforts, Lumino Takapuna is offering the firefighters involved a free exam and x-rays for appointments before Christmas."

Witnesses last night described seeing smoke pouring from the building before an explosion shattered windows on the fifth floor, showering a walkway beneath with broken glass.

Videographer Daniel Hines said the fire started on about the fifth floor and quickly spread. He counted five aerial units with large ladders, dousing the building with water.

The initial call came in about 9pm, he said.

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Another witness said the fire appeared to be confined to one floor, but had spread very quickly.

"It was at least a quarter of the floor up in smoke," said the man, who only wanted to be known as Sam.

"There are fire trucks everywhere and I saw one with a crane going up. There's water everywhere and smoke everywhere. It's crazy."

Emergency services are battling a large fire in a commercial building in Takapuna, Auckland. Video source: Supplied

Another witness said emergency services had blocked off both ends of the street, but he had been walking past the site just as the first of emergency services arrived.

"There was firemen and lots of cops."

A Herald reporter said a "massive hole" had been blown out of the north-west corner of the building on the fifth floor.

The building has a cafe on the ground floor and contains various businesses including Lumino Dentists. Photo / Matt Harvey
The building has a cafe on the ground floor and contains various businesses including Lumino Dentists. Photo / Matt Harvey

It measures about two metres by two metres.

A walkway beneath the building had been showered with shattered glass.

"I can see some lights in there at the moment. It looks like fire are making their way through there to examine it with torches."

She counted about 17 fire trucks and around 15 police officers.

Police had cordoned off the area surrounding the building.

Bhru Soni was working in a nearby building when the alarm activated and he had to evacuate.

Moments later he was standing on the roadside and saw smoke coming from the building.

"We saw smoke for 10 to 15 seconds then we saw fire. We just saw the fire shatter all the windows."

A bystander said there had been an explosion earlier on and another said they had heard a window smashing.

A spokeswoman for St John said they had just been called out to the scene. Photo / Daniel Hines
A spokeswoman for St John said they had just been called out to the scene. Photo / Daniel Hines

Another eyewitness said she saw about 10-15 fire trucks and about the same number of police cars.

"Not in an apartment building though so there shouldn't have been many people inside. It's been going for about 20-30 minutes now and still more emergency services turning up."

A spokeswoman for St John said they were called to the scene, but understood there were no patients.

The fire was extinguished by about 10pm and all occupants are believed to be accounted for.

No flames are visible but smoke can be smelt.

Waitemata Fire service area commander Denis O'Donoghue said emergency services were alerted to the blaze just after 9pm.

"When police arrived they were confronted with a fire coming out of the fifth floor window.

"By the time they got established it was coming out of a number of places in the building and they put an appliance up to the window and they had an attack from fire fighters in the stairwell and attacked it from the inside as well."

Mr O'Donoghue said the fire had been easily extinguished and it was lucky it had not been further up the building where it may have been harder for fire services to access.

Firefighters attend the scene. Photo / Jason Oxenham
Firefighters attend the scene. Photo / Jason Oxenham

He said damage was mainly restricted to level five and minimal damage on level six.

No one was injured in the blaze.

"There will be some smoke damage in the rest of the building as well."

Mr O'Donoghue said the cause of the fire was unknown.

"The temperatures are really hot by that stage.. And there was enough internal pressure that it has pushed out. The glass was coming out of the building and down into the car park behind us."

A cleaner had been removed from the building, Mr O'Donoghue said.

"A cleaner was inside and the cleaner had made their way higher up in the building and we evacuated him."

The building has a cafe on the ground floor and contains various businesses.

Witness Tim Alexander described the fire as "huge".

"We were driving past and just saw lots of smoke. It's huge.

"There's lots of fire trucks - about five or six - and I saw one with the big ladder on it.

"There's loads of police too; we saw about six or seven police cars there."

Auckland periodontist Dr Richard Longbottom, who has a practice on one of the damaged floors, said they were facing a "significant" cost in damages.

"It's on the other side of the building from where I am, but there will probably be a bit of smoke damage. We've got a lot of high-end equipment, so we'll have to see what's gone on."

Asked how much he thought the cost might be, Dr Longbottom acknowledged that because of the nature of their work, in dentistry, a lot of the equipment was already expensive.

"You don't want to know how much our equipment costs, it's a lot of money.

"But at the end of the day, if there's been smoke damage or water damage, [the cost] could be significant."

Dr Longbottom said he had already been in touch with his patients scheduled for appointments tomorrow and would be seeing them at his other practice space in Remuera.

The key thing was that no one had been killed or injured in the blaze, but he would be interested in what fire investigators had to say about the cause of the blaze.