Why isn't New Zealand part of Australia?

It seems an almost heretical question, but it’s gotta be asked! Why are NZ and Australia different countries? You might be surprised to know that we did consider joining up with our mates across the Tasman back in the late 19th/early 20th century, but it never worked out. And that’s just a small fragment of the fascinating history of our Trans-Tasman relationship with our mates across the ditch.