Annoyingly, I think the Apple Store app hasn't got Apple Pay enabled in Australia. I've got Apple Pay set up on my phone, use it often, I just can't buy anything with it in the app. It defaults to my credit card (which, ironically, is the same card as the one used for Apple Pay).
Annoyingly also, the default address keeps dropping out, so I'm having to write it in each time.
Here's hoping both of those things work smoothly come pre-order day. Don't want to waste precious seconds tabbing through the different fields for my address!
But yes, echoing the previous posts, I've found the app and using the "favourite" feature is probably the fastest method. Account, click on the favourite item, add to bag, bag, checkout. It's just a matter of the time it takes to get between these steps, and whether it crashes/goes offline, but I've found the app to be pretty reliable. Much more than the website.