Okay, I'm Australian too, but here's my seven cents worth thanks to the low Oz dollar...
You're creating a problem when any attempt at political correctness, or even a genuine desire not to offend people or perpetuate unpleasant language, diminishes the realism of your characters and their dialogue. In fact, you're possibly doing a disservice by denying such language existed — regardless of any argument about its origins. Imagine someone young reading a novel set in the South circa 19th Century and it didn't contain any of the cruel and racist language used in the day? Would that be right?
If your writing involves perhaps a one-off instance of possible offence, then yeah ... why poke the bear? But if several, even frequent phrases can be offensive, but have genuine historical context — even if that context is to accurately portray a character of the era with their language (remembering, by the way, that many, many people use profanity without having a clue what the words mean) and to maintain the realism of the period, then the best idea would be a brief, heartfelt acknowledgement at the front of the book explaining that you don't condone at all the use of such phrases today, if ever, and that their inclusion is for the sake of authenticity and accuracy.
Acknowledging and identifying any offensive language, and authentically portraying its cruel use, is a lot more powerful message towards removing it today, than pretending it didn't happen.