Have you ever lied about your nationality while traveling abroad?

Nope.

Australians enjoy a good reputation in the parts of the world I have been to (Europe, North America), and I have not been to the parts of the world where they might not (India, Papua New Guinea?). I have been to New Zealand, but any animus there is jocular and reciprocated. The Austria/Australia confusion did not generate any disgruntlement I could detect while in Vienna or Salzburg.

I didn’t broadcast that I was Greek while in Istanbul, but I found that it would not have made a difference: the Thaw has happened. (I still did not feel comfortable volunteering that my father is Greek Cypriot.)

There are more countries in the world where it would make sense to hide that you’re Greek than that you’re Australian, but I haven’t been there either (countries immediately north of Greece).