According to the news daily Kleine Zeitung the explosion completely destroyed an outbuilding next to a family house in Kapfenstein.
The father (57), and his two sons, were reportedly making firecrackers in a warehouse on their property.
The elder son, 33, left the building and shortly afterwards there was an explosion which could be heard a kilometre away, police said.
The father, Josef F, and his 28-year-old son Bernhard are likely to have been killed instantly.
The warehouse was reduced to "just dust and ashes", Maximilian Ulrich from Styrian Police said. Police are still trying to recover the bodies.
The man's 55-year-old wife was in the house at the time and was not injured in the blast. She and her surviving son are in shock and are being cared for by a specially trained team.
A neighbour's house was also badly damaged, according to the Austrian Press Agency (APA), and the fire service received calls from other neighbours whose windows had been damaged by the explosion.
A neighbour told police that the three men often experimented with explosives and that it wasn't unusual to hear loud bangs from the property.