In the US, you're probably looking at anywhere between 5 and about 8% of computers actively used. In Western Europe and Japan you're looking at nearer 3 or 4%. Australia I have no idea.
Because of the relative paucity of language localisations (IIRC OS X is only translated into about twelve other languages, most of which are those of rich Western European countries such as Dutch, French, Swedish, German etc) Apple doesn't really have a much market for its computers in large countries such as China, Russia, and India, as well as the whole of the Arabic speaking world. It's this which reduces its overall worldwide numbers down to typical 2% or so that you read in the press.
If you look at Apple's numbers in countries where they have the local language translation and where they also actively market and sell the Mac, a figure of 4-5% is more appropriate.