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, NT 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,” said Senior Sergeant Garry Smith.
“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,” Snr Sgt Smith said.
Alcohol was a possible factor involved in the stunt, police said.
Paramedics were called although the man had reportedly already taken himself to hospital by the time they arrived.
On Monday police said they believed he would be flown to a specialist burns unit at the Royal Adelaide Hospital.
A Royal Darwin Hospital spokeswoman was unable to confirm the man would be flown to Adelaide but said flights to the specialist unit would indicate he had was seriously injured.
The man’s woes may not be over once his burns heal.
Police said anyone possessing fireworks outside of an approved period faces a $282 fine.
Fireworks in NT can only legally be set off on July 1, known as Territory Day, when the region celebrates its anniversary of being awarded self-governance.
After that day they must be disposed of, but fireworks are often set off for weeks afterwards as people use leftover supplies.