Don't buy any baht in Oz because the rates are lousy and you'll get much higher in Thailand. Some people swear by TCs but I always bring cash + my Oz ATM card. You can withdraw from Thai ATMs using your Oz card, but the fees were recently increased. I currently pay about $AUD18 in fees when I draw 20,000THB from my account (20,000THB is the max) Previously the fees were about $5. Sorry, can't comment on the Amex card.