If you're playing the world map...colonize Australia at around the time you start getting decent naval movement (I'd say just before the Industrial Age, but I'm usually trying to build a massive state on the mainland as Russia so anytime after the Medieval period would work). I found that the AI usually avoids Australia completely, even though there are various citystates on it and around Indonesia. Anyway, bribing those states to support you ensures that they'll keep giving you military units so you won't have to worry about defence. Securing Australia helps later on because usually there's a uranium deposit or two, along with oil and coal. It also is the perfect staging point for an invasion of Indochina and East Asia.
EDIT: Just make sure to bring a couple military units off the bat as well. Usually there's a couple barbarian camps throughout the area, especially if you have Barbarian Invasion on (which I always do).