Yet another example of the white fella's ability to make a random sound and turn it into a derogatory remark. Probably came about as an attempt to mimic one of the many Aboriginal languages (as in "chinese sounds like chingchogwaaafliedlice", "french sounds like ahahaweeweelebaggette",etc...).
Used throughout Australia, mostly by middle-aged to older/elderly Australians, in reference to any Aboriginal or Islander person; and occassionally in reference to other dark skinnned people (Maori, Fijian, Samoan, African) until the white speaker discovers that they aren't Aboriginal. Then they are immediately assumed to be a better class of non-white.
"Well I ran into the wife earlier today, John. They're not boungs, they're Samoans."
"Really? Oh well, that's a new kettle of fish altogether then. Samoans are such lovely, polite people."