To be out of play the ball must be completely outside the boundary line. The field umpire must be informed and, unless a free kick has been awarded, the ball is returned to the position in the field where it went out. If, after a mark or free kick, the player does not put the ball back into play from outside the boundary line, the ball is returned to the field at the point where the original mark or free kick took place.

Aussie Rules - Ball Out Of Play

If a defender, kicking from behind the goal line or behind line, hits a post, time and another kick are allowed.