That is an exaggeration. Car rental and almost all petrol stations accept Amex. So do all major supermarkets and liquor outlets. It's pretty easy to use Amex for 90% of purchases in Australia unless you go to lots of small businesses, non-chain cafes, etc. (There are some weird situations with franchises. e.g. Domino's Pizza accepts Amex online, but an individual shop's equipment may not necessarily be set up for it if you walk in and want to pay them directly. Paying them directly means you can't use online coupons, which are often ~30% off, so usually a very bad idea anyhow.)

I observed lots of DCC in Thailand recently at IHG hotels, but they were good enough to clearly print the surcharge (around 3.8%) at the bottom of the pre-receipt. (Their standard procedure seems to be to show it to you and ask which you'd prefer. I guess if you are dumb enough to see a physical print-out that tells you that you'll be paying more and agree to it, you really have no recourse after the fact.)