Can I use Apple Pay in the US with an Australian AMEX?
Short answer: No, you can't.

Long answer: Apple Pay is, officially, US-only right now. However, it is possible to use Apple Pay in Australia:


  To show the Apple Pay settings, simply open the Settings app on your iPhone (or iPad mini 3 or iPad Air 2), tap on General, Tap on Language &amp; Region, and tap on ‘Region’. Change it to the United States.


However, there is a big caveat:


  While you can go through the setup process, your card must be issued by a participating bank before it can be used with Apple Pay. If you try to add a card from a bank that isn’t yet supported (i.e a card from, or issued in Australia), you’ll simply receive an error.


The Apple support article Apple Pay participating issuers lists the currently-supported banks.

So, while you could use it in Australia with a US-issued card, the reverse is not possible.