Home Postcodes by state WA 6123 Postcode 6123 6123 is the postcode for 2 localities in WA: Mundijong and Whitby. Localities with postcode 6123MundijongWAWhitbyWA Postcode 6123 State or territory Western Australia (WA) Localities 2 Council area Serpentine-Jarrahdale Council area based on Australian Bureau of Statistics data (ASGS Edition 3), CC BY 4.0. Postcode 6122 · Postcode 6124