Gregory Rivers was born in Gympie, Queensland in 1965 and grew up in Katoomba in the Blue Mountains west of Sydney. While studying medicine at the University of New South Wales, he befriended students from Hong Kong and became addicted to Cantopop. In 1987, before he completed his studies, he bought an airfare to Hong Kong and has been living there ever since. His Chinese name is Ho Kwok Wing 河國榮 (Ho meaning River).
(he explains the origin of his Chinese name)
He has been an actor for TVB for many years.
Show reel (there is an intro where he speaks English)
See his blog - An Aussie in Hong Kong
See - interview with Adam Sheik about learning Cantonese; interview with CNN.
Of course, he wouldn't be the only 'westerner' who can speak Cantonese. After all, Hong Kong was a British colony until 1997. It's the perception that Cantonese is a difficult language to learn.
What is interesting about Gregory Rivers/Ho Kwok Wing is that he is probably the most famous Australian in Hong Kong and most people in Australia have never heard of him.