So what makes them different?
Australia is roughly the size of the USA (minus Hawaii and Alaska). Around 25 million people live there.
New Zealand, however, is about the same size as California. In fact, it's 29 times smaller than Australia with a population of 5 million (and, famously, 30 million sheep).
Australia is hotter and drier. 35% of Australia is classified as desert where temperatures can reach 50 degrees Celsius in summer. It's no surprise then that 90% of Australians live within 100km of the coast where temperatures are cooler and the climate is much more pleasant. Most of Australia's big tourist attractions are within 100km of the coast too.
Australia has 4 seasons - spring, summer, autumn and winter except for the tropical Far North, which has a wet and dry season.
New Zealand has a temperate maritime climate. The annual rainfall is a lot higher - which gives rise to lush forests, fertile countryside, mountains, glaciers, and fiords. Plus volcanic landscapes that look like the set of a sci-fi movie. The country has four seasons.
So when you combine New Zealand and Australia you will be set for experiencing a bit of eveything weather wise.