Q: Cannot order Print Products in Iphoto   "The credit card associated with this Apple ID is assigned to the Australia store. If you change the store, you will need to place the order again

Apple stopped the print product support for iPhoto more than a year ago. You can no longer order books from iPhoto. Or are you trying to order a book from Photos?

Are you currently in Australia?  Are your system preferences > Language and Region and your timezone set to Australia?

Open your Account information from the Photos menu > Print Product Store account.

Check and enter again each and every detail, check even the phone number and the country prefix; type the data again, just in case there are invisible characters interfering.

Check also your own card in Contacts and check the formatting.

For the address on your card click the address field pop-up and set it the "Change Address Format".  Make sure, Australia is selected from the list.

and you used the same format on your contacts card to write the zip code and the country name, the state?