Final answer:
During periods of glacial maxima, the lowest sea levels allowed Australia and New Guinea to form a single landmass known as Sahul or Australasia.
Step-by-step explanation:
During glacial maxima, when sea levels were at their lowest, Australia and New Guinea formed a single landmass known as Sahul or Australasia. This occurred because the lower sea levels exposed the continental shelf that these two land areas share, thereby connecting them. The land bridge that formed allowed for the interchange of flora and fauna between Australia and New Guinea, creating unique ecosystems found nowhere else in the world. During these periods of glacial maxima, the combined land mass was much larger than what we recognize today, fundamentally altering the geography of the region.