Question: I'm trying to complete a tax return for an individual who lives overseas, but it not allowing me to use postcode that is 6 character long.

Answer: The State and Post Code fields applies only for Australia addresses. If it’s overseas address it must be left empty.You may just add a postcode to either Line 1 or Line 2 of the address as a workaround.

Note: For foreign addresses State and postcode are not required on tax forms. But ATO still requires that postcodes must be max 4 characters long.