Well, since this has hung around long enough I've had a chance to look
around, and Wilkes (Dictionary of Australian Colloquialisms, 4th ed.) has
one that pre-dates OED's earliest by a few years:
1957 D'Arcy Niland, Call Me When the Cross Turns Over, 52-3 [Description
of the 'Barcoo spews', too long to transcribe]
And Orsman (Dict of NZ Eng), though he doesn't have any printed
occurrences before 1968, quotes Noel Hilliard as saying that "the
variant 'I didn't know if I was Arthur or Martha or General MacArthur'
was current in the 1950s after MacArthur's removal from his Korean war
command by President Truman in April 1951".
So now I'm thinking it really is of Aus/NZ origin. And looking back at
OED's citations, they actually say "orig. NZ". That may be on the
strength of Orsman/Hilliard, but I see that their first citations are
by:
R.Maslyn Williams (Australian) "The Stone Age Island" (1964)
William (Bill) Taylor (NZ) "Plekhov Place" (1971)
First evidence of a spread to the wider world is the 1982 cite from a couple
of Southern African authors.