The 23-year-old man was at a party in the Darwin suburb of Rapid Creek on Saturday night when he decided to let the cracker off, Northern Territory Police said.
“It appears a party was in full progress when a young male decided to place a firework between the cheeks of his bottom and light it,” a news website quoted Senior Sergeant Garry Smith as saying.
“What must of seemed to be a great idea at the time has backfired, resulting in the male receiving quite severe and very painful burns to his cheeks, back and private bits,” Smith said.
According to police, alcohol was a possible factor for instigating the stunt.
On Monday, police said that they believed he would be flown to Adelaide.
A Royal Darwin Hospital spokeswoman was unable to confirm if the man would be flown to Adelaide but said flights to the specialist unit would indicate he was seriously injured.
Police said that anyone possessing fireworks outside of an approved period faces a fine of 282 dollars.
People are legally allowed to light fireworks in NT on July 1, known as Territory Day, when the region celebrates its anniversary of being awarded self-governance.
After that day the crackers must be disposed of, but in reality fireworks are often set off for weeks afterwards as people use leftover supplies.