Not native to NZ, but people release their unwanted pets into the wild. Council's are declaring them as pests, but I have just read that are unable to reproduce here as it's too cold. This one was certainly enjoying the sun on his back today
I used to sell these little guys when I worked as a teenager in a pet shop in Liverpool. We call then Red Eared Terrapins in UK and as you say, they are out of their environment living in these latitudes as they hail from the Mexican Gulf.