I can see where an International buyer would assume it is free post as you have it in the title and also in the description you say free post but don't actually specify that it is for within Australia only.

If I was in your position I would apologise for the confusion regarding postage and explain that free postage is for within Australia only. Also point out to them in the description where it mentions Interrnational Postage.

You can never assume anything on ebay - it is important to make sure descriptions etc are very clear to avoid buyers being confused.