Slang interpretation as Prisoners Of Mother England. Referring to the transportation to Australia from England of convicts considered the dregs of society.
Between 1788 and 1868, about 160,000 people were herded onto ships in Great Britain and 'transported' to Australia with the expectation that due to the 12,000 miles distance they would never return.
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.
Aussie: Fuck off pom, get your shit cricketers off our cricket ovals and give us our fucking ashes - they've been ours since 1989!'
DO NOT CONFUSE POMS WITH CHEERLEADERS!
*poms have much more talent
OTHER GUY: "Yeah I'd like to see them without those uniforms on!"
-- Monty Python