As a general rule, a post-game infraction occurs after the referee gives teams permission to leave the bench under Rule 10.4 (a) which states “At the end of each period, all players must remain on their respective players’ or penalty bench until directed off by the Referee.” Further Rule 10.4 (e) goes on to say, “A Game Misconduct penalty will be assessed to the Coach of any team whose players leave the Players’ or Penalty Bench at the end of a period before they are directed to do so by the Referee and an altercation that results in penalties of any kind is either in progress or begins after the players leave the bench.” 
For example, two players jostling in front of the net that continues after the buzzer and then a fight breaks out is considered a continuation of the game and not a post-game incident.
 
However, a post-game incident can also be defined where there is a significant period of time between the end of the game and an incident occurring. For example, two players proceeding to their respective benches well after the buzzer sounds begin to fight before the referee has released the benches.