The origin of Pom is debated - the abbreviation argument is possible (both POME and POHM) but it never seems to have been a formal abbreviation. Another suggestion is it derived from rhyming slang - Pomegranate for Immigrant, although that has never seemed like a particularly good rhyme to me :)
What I heard from my mates was a different variation of the pomegranate story. The you could always tell a Pom in Oz because he was the colour of a Pomegranate after 10 minutes in the sun.
Although I don’t believe it was the origin of the word, it was definitely true :)