Buy from Apple or retailer but make sure your name and address is on the Tax Invoice (not receipt) as I assume you'll be paying more than $1000. JB Hifi always give me grief over items > $1000 (too hard to get an addressed invoice) so I walk down the street and buy from Digi-Direct.
You haven't got 30 seconds to save $100? :shock:
Download the TRS App from the App Store and enter your details. When you get to the airport, walk up to the TRS counter and stand in the queue on the left (in MEL) with one or two people in it ....... not the one on the right with the thirty-five people who don't have the app. Show the Border Force person your phone with the TRS app open as well as your tax invoice, boarding pass, passport and the credit card you wish your refund paid to.
Officer scans the QR code on your phone, stamps your invoice, swipes your credit card and you're done. THIRTY SECONDS tops!
I know, I did it with a ring + camera + QC25s only 4 weeks ago and it took no more than 30 seconds.