It shouldn't be, but like any other coinage, sometimes things get mixed up. Someone probably went to Indonesia and had left over coinage from the trip and either accidentally or sneakily spent it in Australia.. where it would stay in circulation (probably not that long) until someone noticed it. Canadian and American coins sometimes circulate interchangeably.
New Zealand change probably circulated more freely in Australia, as the coins were all but the same except for the reverse design. (Until NZ changed that in... 05?)