[grey]

Riot accounts are tied to different geographical regions, so I would expect the service doesn't have to span across them, http://leagueoflegends.wikia.com/wiki/Servers

[huac]

Correct, a "North American" account would not be able to message or otherwise interact with a "Oceania" account (the region which contains Australia). That being said, an Australian could register an NA account, but shouldn't expect bearable ping/latency.