The statement that is not true about Australia is:
"It is smaller than the continental United States in size, but has the same population."
This statement is misleading because Australia is actually larger than the continental United States in terms of land area. While both countries have similar population sizes, the U.S. has a significantly larger population than Australia.
The other statements are true: Australia's landmass is entirely south of the equator, most Australians indeed live in coastal cities, and it is considered the smallest continent on Earth.