It was an interchange infringement as I discovered Sunday morning.
If the infringement occurred as Hawthorn was kicking the goal then the goal should have been disallowed and the free kick and 50 metre penalty given.
However if the infringement occurred after the goal was kicked then play has stopped and time on blown. Therefore there is no infringement as long as the extra player is off before the ball is bounced.
Does this not make sense??
