Home Postcode finder SA St Johns St Johns postcode SA St Johns is a locality in South Australia covered by postcode 5373. Postcode 5373 State or territory South Australia (SA) Region Sa Subs Near 1 Add a free St Johns business listing Be the first listing in St Johns Common questionsWhat is the postcode for St Johns?St Johns, SA has the postcode 5373.What state is St Johns in?St Johns is in South Australia (SA).Does postcode 5373 cover more than one suburb?Yes. Postcode 5373 covers 8 localities, so the postcode alone does not identify St Johns. Browse other postcodes in South Australia, or widen the search to every Australian postcode.