Yes you can. The only places that you may have any trouble using your card is Japan and Cuba. Japan because I've heard that ATM machines there may only accept local cards and Cuba due to their idea that Abbey are owned by an American company (MBNA). You shouldn't have any problem at all in Oz, whether it's at an ATM or swiping your card.
Cuba because Visa is a credit clearing house that operates in the US, and on the dollar. To do so means complying with US federal law, and that in turn means not doing business with certain cubans and iraqiis... At least, that's my five cents.