So, I decided to buy Pokemon Sun after all.
I happened to be down at Melbourne Central for unrelated reasons at around 5:20pm on Friday, 18th November. As suspected, Big W had no stock when I meandered in there so late, and they didn't even put a sold out sign out. EB Games had a line snaking out the door, but they price matched Big W for $46. I hope the staff there got paid overtime, because they looked pretty weary.
As usual, here's a brief review of my experience. Please refrain from reading if you have not yet finished the game and still wish to be surprised.
*** Obligatory Possible Spoiler Warning ***