Ringing Domain Stadium and asking someone in the know might be the best way of solving this.
Or if one of our WA members is around there at some point, but that might be a stretch.
EDIT : further to this, I'm sure if you watch the video replay of the whole match., there will be enough chances to see the goal umpire adjacent to the posts - if you assume he (or she if applicable) is about 1.8m tall, you could make a pretty good guess. I just watched highlights from round 8 2015, Freo v Kangas, and the padding in that footage looks about twice the height of the umpire, so if it's still the same, my guess is less than 4m, maybe closer to 3.5, 3.6.
Or if one of our WA members is around there at some point, but that might be a stretch.
EDIT : further to this, I'm sure if you watch the video replay of the whole match., there will be enough chances to see the goal umpire adjacent to the posts - if you assume he (or she if applicable) is about 1.8m tall, you could make a pretty good guess. I just watched highlights from round 8 2015, Freo v Kangas, and the padding in that footage looks about twice the height of the umpire, so if it's still the same, my guess is less than 4m, maybe closer to 3.5, 3.6.