If booked online - probably nothing that you can do, as the booking system will have had a "tick if you agree to the terms & conditions" box - which you have to do to finalise the booking. somewhere in the T&C note will be a phrase along the lines of..."I have checked my itinerary and agree all is fine"...
I know 99% dont read the T&C but we check the box to say we have...
Now, if it was offline with human intervention, then totally different ballgame. You should have been made aware (if on phone then the agent should have read out the Country of arrival), but ALSO, you should have recieved some type of written confirmation.
These days it is generally via email, but will give a full breakdown of all flight data (as well as times/dates etc) - if you have not recieved anything in writing, then you have a strong course of action against the company who booked you.
The "it goes via ATL", or "its flying on US Airways (or whomever)" argument doesnt wash, as it is the travel companies responsibility to ensure you understand what you have booked.
If you have recieved something in writing and either not looked at it, or not noticed, then your out of luck.
The reason the company isnt doing anything about the "error" is because it will cost them a lot to put right (original ticket flight to USA is probably non refundable, and the airline wont refund as a gesture of goodwill, and the differance in cost between MEL FL and MEL AUS is huge.)