I'm going to speculate that every time someone places a lit firework between their buttocks, alcohol is involved (or a new Jackass movie), but it's good to know the police in Adelaide are looking into it. Also, to add insult to likely significant injury, the man is going to be fined for using fireworks outside of the one day allowed by law in the Northern Territory:

A Northern Territory man who set off a firecracker between his buttocks may have to be flown to a specialist burns unit in Adelaide.

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.