Witnesses said the man wheeled the makeshift bomb into the insurance office in the northern city of Darwin and set it ablaze, leaving passers by with cuts, burns and smoke inhalation injuries. Two people were in serious condition.
"Once we cleared the area of smoke we found a shopping trolley with three jerry cans and a strong smell of fuel, and also firecrackers spread throughout the area," a fire and rescue service spokesman said.
"The firecrackers were part of what could loosely be called an explosive device."
A man surrendered himself at the nearby Darwin Police Station shortly after the attack and was taken into custody, said Commander Rob Kendrick.
"I understand that there may have been a shopping trolley wheeled in by the person of interest, but that will be subject to investigation," Kendrick told reporters.
The blast sent shoppers fleeing from a nearby supermarket and prompted the evacuation of the mall in Darwin's main commercial street, as crowds of onlookers gathered.
Shopper Louisa Ainsworth said she was inside the supermarket when she started hearing "all this crashing and banging, sort of like a roof was collapsing and sort of like a little explosion".
"We just went outside and saw all the smoke and people screaming and running out of TIO (the Territory Insurance Office)," Ainsworth told public radio.
Another witness, Charmaine Burton, said she heard "15 to 20 repetitive (bangs) almost like gunshots" and there was "a diesel fuel sort of smell" coming from the site.
Police ruled out an extremist attack, saying the culprit appeared to be an unhappy customer of the insurance company.
"I can confirm he was a dissatisfied claimant," Commander Colleen Gwynne said. "This is not a terrorist incident.
"In hindsight, I think we're very fortunate all round that we haven't lost any lives," she added. "This is an isolated incident by a dissatisfied customer, and it was an extremely callous act on his part."
Ambulance officials said 13 people were taken to Royal Darwin Hospital with injuries ranging from "minor cuts to burns and smoke inhalation", while another two travelled there independently.
Witnesses said they saw a man push a shopping trolley loaded with cans of fuel and fireworks into an insurance company office inside the mall.
"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," an unnamed witness told public radio.
"He said he stood there for a minute and watched it catch fire and then took off."
TIO chief executive Richard Harding said the company had not received any threat or warning ahead of the blast.
"At this stage it appears to be a random event, a person has walked into the branch and ignited some petrol," he told reporters. "Other than that we have no details as to what has caused the explosion."
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