To get around that when using flat rate shipping you can state the same service for both Canada and international. For example, I use the term Standard Int'l Shipping for Canada and then use it again for any other country outside of the U.S.Then, the Australian buyer will only see the price to their location. An example is item 271398805260
You shouldn't be stating that you are using USPS first class to Canada unless you actually are going to the U.S. and shipping from there. If the buyer doesn't look at your location, they might assume you are an American seller which isn't necessarily a good thing as they might prefer to buy from within Canada.