Home Postcodes by state WA 6403 Postcode 6403 6403 is the postcode for Grass Valley, WA. Localities with postcode 6403Grass ValleyWA Postcode 6403 State or territory Western Australia (WA) Localities 1 Council area Northam Council area based on Australian Bureau of Statistics data (ASGS Edition 3), CC BY 4.0. Postcode 6401 · Postcode 6405