Home Postcode finder WA Middlesex Middlesex postcode WA Middlesex is a locality in Western Australia covered by postcode 6258. Postcode 6258 State or territory Western Australia (WA) Region Wa Subs 3 Sth Rd Add a free Middlesex business listing Be the first listing in Middlesex Common questionsWhat is the postcode for Middlesex?Middlesex, WA has the postcode 6258.What state is Middlesex in?Middlesex is in Western Australia (WA).Does postcode 6258 cover more than one suburb?Yes. Postcode 6258 covers 22 localities, so the postcode alone does not identify Middlesex. Browse other postcodes in Western Australia, or widen the search to every Australian postcode.