Answer:
The main reason is that the USA is in the Northern Hemisphere & Australia is in the Southern Hemisphere.
Explanation:
It has all to do with the tilt of the earth as it revolves around the Sun. Summer happens in the hemisphere tilted towards the Sun, and winter happens in the hemisphere tilted away from the Sun.
Now,in June, in the USA is tilted towards the Sun, the days are getting longer & the nights shorter, so it is getting warmer in the USA as they enter their Summer. It is the exact opposite in Australia.
Now, in June,in Australia is titled away from the Sun, the days are getting shorter & the nights longer, so it is getting cooler in Oz as they enter their Winter.
