Although it appears that going from Australia to North America, there is nothing but the great Pacific Ocean underneath. However the truth is (as Federico mentioned) that there are many airports along the way which can allow an A380 to land there, and many more close enough that an A380 can divert to in case of an emergency.
During the early days of this flight (Qantas 8 to be exact), it had to stop at Noumea, New Caledonia for a fuel stop as the pilots thought that it would be safer to do that.
Wikipedia has a list of diversion airports in Pacific. If you turn on the options to view nearby airports on FlightAware and zoom in on the map, you can find many airports.
It should also be noted that not all emergencies require an immediate landing.