Tunesmith's "Yank", in my experience, is the preferred choice; however, the word "yankee" would be used in the following sentence, " He drove a big Yankee car" is half similar to the Oz experience.

During WWII there were many Americans in Oz and, while many were warmly welcomed (in as many senses as were possible) many Australians had 'less than complete' acceptance. Irrespective of where they came from Americans were referred to as "Yanks" and this led to the rhyming slang term for an American as a "septic tank".

I never heard the phrase " He drove a big Yankee car" in Oz. It was always "He drove a Yank tank"; the "big" was always implied and understood.

Cheers, Rowan