Well it’s a bit rude, but basically Seppo or Septic as in Septic tank - Yank.
When the guys in the factory would give me a hard time and ask if I minded being called a Pom I’d say no as long as they didn’t mind me calling them a bunch of bread thieves or Skippies (Skippy the Kangaroo).
Basically I think the rule of thumb in Australia is the more curse words and the ruder the Knick name the better you get along.