The only two continents located entirely in the Southern Hemisphere are Antarctica and Australia, according to About.com. Wikipedia explains that the entire mainland of Australia lies in the Southern Hemisphere, which qualifies it to be considered one of the two continents located in that hemisphere.
About.com states that the Southern Hemisphere represents the southern half of the Earth, beginning at the equator and continuing all the way south to the South Pole. It contains more water and fewer land masses than the Northern Hemisphere, with water accounting for over 80 percent of the hemisphere. The significant amount of water in the Southern Hemisphere creates an overall milder climate than that found in the Northern Hemisphere.