Blizzard should adapt the time zone thing from Rockstar where they release dlcs on the same day world wide.
If they did that, Australia will be playing it right now
For Sydney it's 4:30 I believe because Halloween dropped at 4:30 last time
I will add to this though that it may be out before this as they've often put out events as early as 5pm GMT so I would just keep an eye out from 5pm onwards which is in about 3 hours