What is the zip code for Australia An American company keeps asking me for zip code and won't accept the Australian postcode?

Australia uses a system of "Post Codes" consisting of 4 numeric characters.

I do not expect that Australia Post will change the system it has been using for over 40 years to accommodate the inadequacies of a foreign corporate computer system.

You might suggest that they insert a zero "0" in front of the four Post Code digits.