The earth is NOT flat.
A flat earth cannot, by any stretch of the imagination, explain the fact that there is a set of northern circumpolar constellations - draco, ursa major, ursa minor, cassiopeia, and cephus which never set in the northern hemisphere. None of these constellations is visible at any time of the year from Australia or any points in the southern hemisphere. The reverse of that is there is a set of southern circumpolar constellations visible from Australia and other southern hemisphere countries - the southern cross, carina, and centaurus which are not visible at any time of year from the northern hemisphere, unless you drop below the 30th degree of latitude.
A flat earth cannot explain polaris being the pole star in the northern hemisphere but not the southern hemisphere. Right now, there isn't a relatively bright star like polaris in the southern sky, unless you're talking about Sigma Octantis.