Home Postcode finder SA Hay Flat Hay Flat postcode SA Hay Flat is a locality in South Australia covered by postcode 5204. Postcode 5204 State or territory South Australia (SA) Region Adelaide Add a free Hay Flat business listing Be the first listing in Hay Flat Common questionsWhat is the postcode for Hay Flat?Hay Flat, SA has the postcode 5204.What state is Hay Flat in?Hay Flat is in South Australia (SA).Does postcode 5204 cover more than one suburb?Yes. Postcode 5204 covers 10 localities, so the postcode alone does not identify Hay Flat. Browse other postcodes in South Australia, or widen the search to every Australian postcode.