Home Postcode finder WA Williamstown Williamstown postcode WA Williamstown is a locality in Western Australia covered by postcode 6430. Postcode 6430 State or territory Western Australia (WA) Region Wa Subs 1 G Flds Add a free Williamstown business listing Be the first listing in Williamstown Common questionsWhat is the postcode for Williamstown?Williamstown, WA has the postcode 6430.What state is Williamstown in?Williamstown is in Western Australia (WA).Does postcode 6430 cover more than one suburb?Yes. Postcode 6430 covers 14 localities, so the postcode alone does not identify Williamstown. Browse other postcodes in Western Australia, or widen the search to every Australian postcode.