New Zealand has by far been my favorite country to travel to so far. The people are amazing, the southern alps are incredible, and I've felt safer there than in my own home country. The only real downside I noticed is that the nature and wildlife isn't really that varied. Don't get me wrong, I absolutely loved seeing the Kea, Kaka, Tui, Fantails, and walking through the rainforests, alpine meadow and tundras. However, a rainforest in bay of Islands is practically identical to the one in Waikato, or Raikiura, or West Coast (though they have Nikau palms). And when hiking you mostly only come across Fantails, Tui and the occasional Weka.
While New Zealand truly shines in terms of the sheer scale and how dynamic the landscape is for such a small country, Australia's nature and wildlife is on a whole other level. You have the primal rainforests of the daintree, Great Barrier Reef, savannahs, eucalyptus forests, and much more. Every time I move on to another place I'd come across animals that I've never seen before. Crocodiles, cassowaries, tropical fish, spiders, platypuses (platypi?), ibises, wallabies, kangaroos, etc. etc.
Where New Zealand is truly stunning in terms of geography with its mountains, geothermal activity and incredible scenic drives, Australia is truly stunning in terms of nature, wildlife, and different climate regions. If you ever find yourself in a position to travel to be able to travel to one of them, definetly consider visiting both at the same time! They require a lot of time to properly explore, but it is the trip of a lifetime.
Metaphorically, I like to compare the landscape of New Zealand to a small child. It is dynamic, highly active and ever changing. Australia's landscape is comparable to an old grandfather. It is stable, ancient, and full of fascinating stories for whoever is willing to listen.