Because this is the most frequent question we are asked. You could not be an Australian Astronaut until 2018 because we didn’t have a space agency. Nonetheless when the Space Agency begins it would not be a sensible financial ROI to pay for one in the first years. Even most established space agencies do not have human spaceflight programs. The best way (as shown by Paul and Andy) is to start working on the NASA requirements, move to the US and gain citizenship to apply over there.