A dissatisfied customer pushed a shopping trolley full of fuel and fireworks into a Darwin bank, setting off an explosion that injured at least 15 people.

After pushing the trolley into the TIO Insurance and Banking office in the Woolworths shopping centre in Darwin's CBD about 11am (CST) on Wednesday, the man handed himself in to police.

Major Crime Commander Colleen Gwynne said the man had made multiple threats against the branch previously.

"There has been some history between the person that we have in custody and TIO," Ms Gwynne told reporters late on Wednesday.

"He is a dissatisfied claimant and there has been involvement with police just recently."

She said the nature of his complaint was not yet known but that security had been increased at the branch since he began making threats.

"It's clear that he wasn't happy with a service that he was being provided," Ms Gwynne said.

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

A team of 50 emergency service personnel would investigate the incident further before charging the 44-year-old Darwin resident, she said.

"We feel very fortunate we haven't lost any lives in what we see as an incident in the upper echelon of seriousness."

National Critical Care and Trauma Response Centre executive director Len Notaras said 15 adults had been transferred to Royal Darwin Hospital suffering a range of injuries, including smoke inhalation and burns.

The worst-affected patient suffered burns to up to 20 per cent of their body.

"Five of the victims are in the high-dependency unit at Royal Darwin Hospital," Dr Notaras told reporters.

"Their condition is stable in all cases, but in the same token three have needed to go to the operating room to have the burns debrided or cleaned."

A handful of victims had been discharged.

Dr Notaras said he expected more people would seek medical treatment overnight. "They've been through a very traumatic event."

He said the explosion represented a smaller scale domestic version of the first Bali bombings.

"(The fact) that the consequence hasn't been more tragic is a credit to all of the responders," he said.

"This is the first formal response of a mass casualty and it (the centre) responded splendidly."

Northern Territory Chief Minister Paul Henderson commended the actions of emergency services and medical staff, as well as the actions of the public.

"Everybody's kicked into action to provide care and comfort to the people who have been affected," he told ABC Radio.

"The shock of this incident has reverberated around the city."

Mr Henderson said he did not believe the incident was an indication of the financial concerns and frustrations affecting the community.

"I can't speculate as to what has gone on inside this person's head."

Woolworths issued a statement on Wednesday afternoon saying it had reopened the Darwin store.

The explosion caused extensive damage to the TIO office.

Charmaine Burton, who was working in a nearby office, said she heard several loud, evenly spaced bangs that sounded like semi-automatic rifle fire.

"It was just hysteria, everyone was just in panic and I just hope everybody's OK," she said.