If you don't want to sell overseas, it's not enough just to not offer international postage in your listings. You have to physically block them as well. You may have to use the website in your phone as opposed to the app.

Go up to the top of your My eBay page in the 'classic site' and click accounts, then site preferences. Scroll down to Postage Preferences, then click Show. A list of options will appear. Click Edit next to Exclude Postage Locations from your Listings. If you don't want to post to any location overseas, make sure every box is ticked. You don't need to expand them.

Tick Apply To All Current Live Listings, then apply.

I'm not sure if clicking this link will get you straight there, but try this before the above. It might be quicker on the phone if it works (and easier!!!) http://cgi5.ebay.com.au/ws/eBayISAPI.dll?ExcludeShippingList

Once you have ticked all those boxes, people will still be able to see your listings if they visit the AU site, but it will show "Does Not Post To <insert country>" where the postage should show. It will also stop them buying if they try.