from random website
From Rosemary Wetherall: "Is pom short for Port of Melbourne (where the ships docked), Prisoners Of her Majesty, as they were convict ships, or did we all really look like a cargo of pomegranates when we caught the sun? Or is it simply rhyming slang for immigrant?"
[A] You've done a great job of listing many of the explanations that one comes across for the origin of this Australian term for British immigrants. You could have added a possible derivation from Prisoner of Mother England, from the common naval slang term for Portsmouth, Pompey, or from pommes de terres for potatoes, much eaten by British troops in World War One, or an abbreviation for Permit of Migration. All of them except your last two, I have to tell you, are folk etymology (which, for some reason I've never understood, loves to invent origins based on acronyms).
Part of the reason for all these theories growing up is that there was for decades much doubt over the true origin of the expression, with various Oxford dictionaries, for example, continuing to say that there is no firm evidence for the pomegranate theory. That origin was described by D H Lawrence in his Kangaroo of 1923: "Pommy is supposed to be short for pomegranate. Pomegranate, pronounced invariably pommygranate, is a near enough rhyme to immigrant, in a naturally rhyming country. Furthermore, immigrants are known in their first months, before their blood 'thins down', by their round and ruddy cheeks. So we are told". You will note that he had to explain the pronunciation that we would now take to be the usual one: in standard English it used not to have the first "e" sounded, with pome often rhyming with home.
It is now pretty well accepted that the pomegranate theory is close to the truth, though there's a slight twist to take note of. H J Rumsey wrote about it in 1920 in the introduction to his book The Pommies, or New Chums in Australia. He suggested that the word began life on the wharves in Melbourne as a form of rhyming slang. An immigrant was at first called a Jimmy Grant (was there perhaps a famous real person by that name around at the time?), but over time this shifted to Pommy Grant, perhaps as a reference to pomegranate, because the new chums did burn in the sun. Later pommy became a word on its own and was frequently abbreviated still further. The pomegranate theory was also given some years earlier in The Anzac Book of 1916.
Whatever your beliefs about this one, what seems to be true is that the term is not especially old, dating from the end of the nineteenth century at the earliest, certainly not so far back as convict ship days.