Paramedics treated 21 people at the scene, an ambulance spokesman said. Most suffered breathing problems but five have serious burns.
“Police responded to reports that a man had allegedly set fire to a Springvale Rd bank,” Victoria state police said of the incident in the Melbourne suburb.
“The man has been taken to hospital in a serious condition under police guard.”
Eric Sleuriot, who was nearby, told the Melbourne Age newspaper: “It’s bloody awful. I just can’t believe it.”
The Commonwealth Bank, Australia’s biggest, said the branch would remain shut for the rest of the day, with its response team on site.
“We are working closely with local authorities and emergency services,” it said in a statement.