Auckland, New Zealand and Madrid, Spain are a near antipodal pair. Both are located at the diametrically opposite points to each other on Earth’s surface. Therefore, we can fly from Auckland in any direction and arrive in Madrid after traveling a roughly similar distance, regardless of the direction taken.
Any flight between an antipodal pair has a similar duration even though the different flights have different directions. This fact is only consistent with spherical Earth and cannot possibly be explained if Earth is flat.