Four digit post codes were introduced by the Post-Master General’s Department in 1967. Prior to then, from 1928, Melbourne used a system of numbered postal districts based on that used in London, combining a letter or letters indicating direction and a number based on distance from the GPO.
Carnegie’s designation was SE9. Our neighbours across Dandenong Road, when they were happy to call themselves Carnegie North rather than East Malvern (see here), had a different designation – SE10. Up until about a year ago there was an old street sign, on the corner of Grange and North Roads, that bore SE14, being the postal district number for Ormond.
A full listing of all the old postal district numbers is on Wikipedia (see here).