Does anyone have a good map of which online dictionary will most likely apply to a given English speaking community?
I'd say the Oxford English Dictionary is the definitive British English dictionary. Merriam Webster's might be the definitive American English dictionary.
Using Gomez's technique, it seems that the Macquarie Dictionary is the one preferred in Australia.
Here's a pseudo-scientific, fascinating look at dictionary use by region using Google Trends. For clarity, each phrase was crafted to return a dictionary as the first hit in (American, Western Washington) Google.
Especially interesting are the usage statistics for the Philippines and India, though the former seems heavily skewed by Tagalog search results.
The languages vary tremendously in both spelling and meaning, in some areas (such as cookery and food) more than others. The language is relatively common and certainly in the modern age with the perfusion of international television, we understand many of the differences orally due to exposure. This does not mean we see the spellings everyday, or have a firm grasp of the usage differences, particularly when obscure.
Using a dictionary tailored for another region than your own is unwise at best. The languages are not the same, though are mutually corruptive.