Home Postcode finder SA Duncan Duncan postcode SA Duncan is a locality in South Australia covered by postcode 5223. Postcode 5223 State or territory South Australia (SA) Region Sa Subs Near 2 Add a free Duncan business listing Be the first listing in Duncan Common questionsWhat is the postcode for Duncan?Duncan, SA has the postcode 5223.What state is Duncan in?Duncan is in South Australia (SA).Does postcode 5223 cover more than one suburb?Yes. Postcode 5223 covers 30 localities, so the postcode alone does not identify Duncan. Browse other postcodes in South Australia, or widen the search to every Australian postcode.