At least 13 people have been hospitalised after a man pushed a trolley containing petrol and fireworks into the doorway of the TIO Banking office in the Darwin CBD.
Police say six people were taken to Royal Darwin Hospital by ambulance and a further seven were transported there by car.
At least two are in a serious condition.
Northern Territory Fire crews arrived at the Woolworths shopping complex shortly after the explosion to find three jerry cans on the floor of the TIO Banking office.
Major crime investigators will hold a media conference on Wednesday afternoon.
It is understood a police officer was among those hospitalised due to smoke inhalation.
The man who pushed the trolley into the complex surrendered himself to Darwin police not long after the explosion at 11am (CST).
NT fire brigade Darwin station officer Dave Lines told AAP the supermarket was ablaze when fire crews arrived but it was already mostly controlled by the centre's sprinkler system.
He said it appeared someone had pushed a shopping trolley containing three jerry cans of petrol and a cluster of fireworks into the building.
He said the explosions caused a relatively significant amount of damage to the supermarket and he expected it would need to be refitted.
Charmaine Burton, who was working in a nearby office, said she heard several loud evenly-spaced bangs and that it sounded like a semi-automatic rifle.
She said smoke was billowing out of the doors of the supermarket.
Police were at the doors calling "if you can hear me, come to my voice", she said.
There was an accelerant type smell, like "kero", or a diesel fuel sort of smell, she said.
Ms Burton said people were in panic after the incident.
"It was just hysteria, everyone was just in panic and I just hope everybody's OK."