As one of many ex Air Nelson cpt's who left around 7 yrs ago due to lack of career progression I can safely say it was one of the best things I have ever done. There is a whole lot of opportunity out there. Many of us who left are in the left seat of medium to heavy jets( some in training roles) and will nerver look back.
Having to leave your home country in pursuit of your career is not the end of the world. Air new Zealand is a big fish in a very very small pond, there are by far better choices out there.