ISP's usually use "backhaul" providers to go long distances, especially from country to country. That's probably where your problem is. You need to test your speed from Australia to the US to see what kind of speeds you are getting. A test site in Australia for speed won't give you an accurate speed for going overseas.
And if that's the case, there is usually something your ISP can do to fix the problem, although it may cost them some money.