Other than the Player kicking the football into play, all
Players must immediately vacate the area within 5 metres
of the Goal Square
Where the field Umpire is of the opinion that a Player from the
attacking Team:
(i) is within 5 metres of the Goal Square at the time when a Player
from the defending Team is Kicking the football back into play
and has had sufficient time to vacate the area;
(ii) has prevented or attempted to prevent the Player from the
defending Team from kicking the football back into play; or
(iii) has otherwise delayed play
The field Umpire shall award a Free Kick to the defending Team
and a fifty metre penalty from the Kick-Off Line.
Looking at those rules and the incident I'd say the umpire got it right as Campbell didn't actually interfere with Mules at all and didn't have sufficient time to vacate the area as he was running.
Mules just made a shocking decision and followed it up with a more shocking kick!
There was a lot weirder decisions during the day resulting in the Bays winning the free kick count by 7 which I found bamboozling since we were first to the ball all day and usually playing in front.