Six people are in a critical condition and 27 people were injured after a fire at a Commonwealth Bank branch in Springvale in Melbourne's southeast.
Police said a man set himself alight after walking into the bank with a flammable accelerant, about 11.30am.
Two men in their 20s, a man in his 60s, a man in his 70s, a woman in her 20s and another man were taken to the Alfred suffering serious burns.
Ambulance Victoria said 21 other people - including a toddler and a person in their 80s - were treated for breathing problems and taken to hospital.
Acting Inspector Jackie Poida said it was "too early to tell" what the man's motive was.
"He had with him some sort of accelerant, he walked in to the bank and he lit that accelerant which caused some sort of fire within the bank," she said.
"He set himself alight with that fire and there was a number of other people in the bank at that time that received some injuries as well."
"He left the bank, police attended shortly after and assisted him."
The man was discovered by police at the back of the bank.
Police did not know much about the offender.
"We haven't had a chance to speak with him, he is injured and he is being attended to at hospital," Acting Insp Poida said.
Asked whether the attack was terror-related, Acting Insp Poida said it was "too early to tell".
Despite reports from witnesses of a possible bomb, CFA operations officer Paul Carrigg confirmed a "flammable liquid" had caused the blaze.
"A flammable liquid could have caused an explosion," he said.
Firefighters had not found a petrol canister but Carrigg said they "haven't gone looking".
He didn't offer any information as to how the liquid was transported but did say given the severity of the fire it was fortunate no one had died.
"It's very amazing, very lucky no one lost their lives."
Caltex Australia spokesman Sam Collyer said the petrol station had been closed since this morning's explosion at the bank.
The Herald Sun understands the man bought the petrol at the Springvale Caltex before taking it down the road and igniting himself on fire.
"The site is currently closed but the we are assisting police with their inquiries, and will continue to do so wherever needed," Collyer said.
The arson squad is leading the investigation.