A "disgruntled claimant" was responsible for a fire attack on an insurance office in Darwin that left 15 people injured, police say.

They have described the incident as isolated and terrorism has been ruled out.

Police say a 44-year-old man wheeled a shopping trolley containing three jerry cans and fireworks through the front door of the TIO (Territory Insurance Office) premises in Cavenagh Street about 11:00am (ACST) on Wednesday.

They say he has previously made threats against the company.

"There was a rapid burning fire in the TIO building caused by a male person who entered the office," Commander Colleen Gwynne said.

"I can confirm he was a dissatisfied claimant.

"This is not a terrorist incident.

Later, she said: "There has been some history between this person that we have in custody and TIO.

"He's a dissatisfied claimant and there has been involvement with police just recently and TIO to try and work together to deal with some of the issues arising out of some of the threats that he's been making.

"This particular individual has carried out threats, not only in relation to TIO, but other individuals and agencies as well."

Fifteen injured people have been treated at the Royal Darwin Hospital. Five people are in the high dependency unit, but head of the Trauma Response Centre at the Royal Darwin Hospital, Len Notaras, says none are critical.

"All of them are awake, but by the same token they've been through a very traumatic experience," he said.

The injured include a policeman who entered the smoke-filled office to rescue people.

He is suffering from smoke inhalation.

'Acts of heroism'

"With these incidents your instinct is to save lives," Commander Gwynne said.

"And it doesn't get more serious than an incident such as this when you have innocent people that are taken by surprise.

"So you always get acts of heroism and that's what we did hear."

Police say the alleged attacker surrendered to them after the incident.

"I can tell you he is a Darwin resident," Commander Gwynne said.

"As I said, he is dissatisfied with TIO.

"For whatever reason I don't know.

"He seemed to respond by carrying out what seemed to be a very callous act."

Dr Notaras said the victims suffered burns ranging from superficial to serious.

"We expect that their condition will remain stable, and they may well need to have additional oxygen to assist in their breathing," he said.

"And in at least one, possibly two, cases they may need to go to theatre, the operating room, to have the burns of the upper limbs, the arms, cleaned."

The TIO office is inside Darwin's CBD Woolworths complex. The bomb squad was on the scene shortly after the incident.

The entire shopping complex was evacuated as were neighbouring properties.

'Lit it up'

Emmanuel Gerakios, whose cousin works at TIO, said his cousin suffered smoke inhalation during the incident.

"Someone came in, they were probably distraught or displeased with what happened," he said.

"They brought in two jerry cans in a shopping trolley and they actually poured it all over the floor and lit it up and they walked outside, they ran and everything went up in smoke."

A witness who was in the car park said his relative saw the events take place.

"My son-in-law came out, he turned around and saw this tall skinny guy walk in with a trolley full of fuel and firecrackers and set it off.

"He said he stood there for a minute and watched it catch fire and then took off."

Witness Charmaine Burton was in a meeting nearby when the attack happened.

She told the ABC that she heard "15 to 20 repetitive (bangs) almost like gunshots".

She said the smoke inside the TIO office was "completely black" and there was a strong smell of diesel or kerosene.

She described how people inside the building had to follow each other's voices to safety because they were unable to see because of the smoke, with one man calling out "if you can hear me follow my voice".

TIO's chief executive, Richard Harding, says details about the man's motive are not yet known.

"Of course it's absolutely concerning that he's a disgruntled complainant.

"At the moment though, we need to work with the police to make sure that [they] understand fully what the background to his actions is."

Five ambulance vehicles were on the scene and Darwin hospital's burns trauma unit was put on standby.

Louisa Ainsworth was shopping when she heard the explosion.

"I was shopping at Woolworths and then just started hearing all this crashing and banging, sort of like a roof was collapsing and sort of like a little explosion," she said.

"We just went outside and saw all the smoke and people screaming and running out of TIO."

The Northern Territory Chief Minister, Paul Henderson, said he was shocked by the incident.

"Darwin remains a tight-knit community and the shock from this incident has reverberated around our city," he said.

Additional reporting: Eleni Roussos, Louisa Rebgetz and Aislin O'Connor