Why: On Eric Bana's Wikipedia page:
In a cover story for The Mail on Sunday, he told author Antonella Gambotto-Burke that his family had suffered from racist taunts, and that it had distressed him. "Wog is such a terrible word," he said.His father is Croatian and his mother is German.
Answer: In Australia (where Bana grew up), it's an ethnic slur to denote immigrants of eastern or southern European origin. While it is increasingly used as an affectionate term - especially by those the term is used to describe - it is still very much an insult to those who grew up in Australia in the 1950s to 80s.
In Britain, "wog" refers to dark-skinned, non-white people from Africa or Asia.
In Scientology, it means "Worthy Ordinary Gentleman" and is used as a disparaging term toward non-Scientologists.
The More You Know: The slang term may have come from Golliwogg (below), a blackface minstrel doll / character from a book published in 1895. Alternatively, it might have come from the maritime term pollywog, used for soldiers the first time they crossed the equator.