Home Postcodes by state SA 5290 Postcode 5290 5290 is the postcode for Mount Gambier, SA. Localities with postcode 5290Mount GambierSA Postcode 5290 State or territory South Australia (SA) Localities 1 Council area Mount Gambier Council area based on Australian Bureau of Statistics data (ASGS Edition 3), CC BY 4.0. Postcode 5280 · Postcode 5291