>Also, island nations still have internal traffic. What's different about the borders between parts of Australia that allowed them to be closed when European borders had to stay open?

Australian states' borders aren't like those between New York and New Jersey (or Colorado and Wyoming), or Belgium and the Netherlands. There are a handful of highways that can be and were completely blockaded, and otherwise the borders are completely empty desert. (Even on the highways, there are dire warning signs about having sufficient water/gasoline/supplies in the more remote parts.)