Home Postcode finder SA Williamstown Williamstown postcode SA Williamstown is a locality in South Australia covered by postcode 5351. Postcode 5351 State or territory South Australia (SA) Region Sa Subs Near 1 Add a free Williamstown business listing More business listings in Williamstown Businesses in WilliamstownWilliamstown Primary School5 Margaret St · 08 85246289 Common questionsWhat is the postcode for Williamstown?Williamstown, SA has the postcode 5351.What state is Williamstown in?Williamstown is in South Australia (SA).Does postcode 5351 cover more than one suburb?Yes. Postcode 5351 covers 7 localities, so the postcode alone does not identify Williamstown. Browse other postcodes in South Australia, or widen the search to every Australian postcode.