He needs to separate Australian and overseas sales because he does not pay GST on international sales (just like international postage is also GST free).

This is very easy to do.

Go to paid and posted, select all and download as a csv file. You can then sort your national and international sales. However you need to do this report monthly as it only keeps the past 60 days which wont help you complete your BAS if you haven't done it yet this quarter.