Either comes from 'prisoner of mother England' or pomigranite - a reddish coloured fruit that native Australians (Aboriginals) thought had a similar colour to the skin of sunburnt Brits.
Not meant to be an insult (as some English think for some reason), merely a nickname for our less-tanned former rulers. Nicknaming everything is very Australian.
Pom: 'Oi, convict, get your shit stars off our flag!'
Aussie: Fuck off pom, get your shit cricketers off our cricket ovals and give us our fucking ashes - they've been ours since 1989!'