You do not have to fly into the northern hemisphere if you are flying from Australia. The only reason that you would absolutely need to fly into the northern hemisphere from Australia would be if your final destination were located somewhere within the northern hemisphere. Australia to Europe, for instance, would absolutely require you to fly into the northern part of the globe, because all of Europe is located above the Equator. However, if your flight were to take you from Australia to New Zealand, flying into the Northern Hemisphere would make your trip unnecessarily long. That's because New Zealand is already located south of Australia and within the southern hemisphere. The same is true of the entire continent of Antarctica. Its entirety is located south of Australia.