> Oxborough, due west of Norwich once included a settlement of returned Australian convicts.

Interesting, of course. But it's likely enough that whoever created the lyrics simply wanted to write an erotic song set in the romantic South Seas. Ah, the Hawkesbury banks! Why not? Particularly if it was written in Sydney.

Surely this is just as probable as any suggestion that the song originally referred to the "Oxborough" banks.

FWIW, a search of Google Books for "Oxborough + Australian" fails to confirm the existence there of a settlement of returned lags.