"When played, it was a tradition that team members would pull down their trousers and dance," court documents stated.
After the song, the victim went to continue drinking at a table with his mates, several of them still bottomless.
Keach grabbed a pool cue from a wall rack and "with a forceful upwards motion stabbed [it] into the victim's buttocks area".
The result of the blow caused an 8cm laceration to the man's anal canal.
He felt a sharp stabbing pain on impact and turned to see Keach with a smile on his face, the court heard.
The victim immediately went to the toilet to assess the damage while the defendant continued drinking with friends.
After struggling to stem the heavy bleeding, the victim returned to the bar area and punched Keach twice in the face.
Police, who happened to be at the pub, broke up the resulting scuffle.
The victim was taken to Balclutha Hospital and then transferred to Dunedin where he underwent surgery to the lining of his rectum and underlying sphincter muscle.
He was discharged a day later but could not work for a couple of weeks.
The surgeon who dealt with the victim said "significant force" would have been required to cause the wound and without treatment it could have led to infection and incontinence.
Keach told police that he had not intended to harm the man and was unaware he had caused injury.
He described the incident as "a bit of a laugh", the court heard.
The defendant told officers he had consumed between five and seven beers at the time of the incident and described his act as "a firm poke".
Judge Kevin Phillips convicted Keach and remanded him on bail until December.
An assessment for electronically-monitored sentences would be carried out but the judge stressed that did not mean the man would evade incarceration.
At defence counsel John Westgate's request, Judge Phillips deleted Keach's curfew and allowed him to enter licensed premises again.
He was still barred from consuming alcohol, however.
