Actually the Canadian dollar is abbreviated as CAD and not CDN.
You are correct about the AUS
part of Australia. With time, many people started abbreviating the word Australia with Oz and today you will commonly hear people refer to Australia as Oz. It became part of the vast collection of "Aussie slang" words.
We experience it quite a lot in South Africa as well. With 11 official languages there is much to choose from and you will often hear black South Africans using English or Afrikaans words in their sentences whenever they don't have a similar word available in their particular language. However, this is not bound to black South Africans. White South Africans also have their fair share of slang - you will for instance almost never hear the word "barbeque" used in South Africa. We refer to it as a "braai".
Point being, its slang and not "proper language", but what the hell! We love to use it!
When you are doing stuff that nobody has done before, there is no manual – Kevin McCloud (Grand Designs)