You must keep a return ticket if you are on a visitor visa to New Zealand. My wife went to check in at Cochin International Airport without a return ticket. She was on Singapore airlines and the front staff asked to show the return ticket. She tried explaining that she will move to a work visa soon as she is going to stay with her husband (me). The staff still asked to show the return ticket. I soon book a return ticket and then only she was able to board the flight. The funny thing was that, the Immigration officer in New Zealand (Christchurch) don’t ask for a return ticket proof.