Saved? Us?
More like invaded.
It was coined by as cockney rhyming slang during WWI, I believe because of the perceived similarity between the two objective nouns. i.e.; They are both full of excrement.
It was then adopted in Australia during the WWII invasion and shortened to it's more common form over here; Seppo.
http://alldownunder.com/oz-u/slang/rhyming.htm