I have to wait six hours for connecting flight at CAN to NZ. Are there facilities to shower or rest?

Well, you can take some rest in the public area of the airport. But the shower facilities are only provided by VIP Lounges. They just serve for first-class cabin passengers and business class passengers.
So if you want, you can stay at the hotel nearby the airport. Airport Hotel is recommended. It provides comfortable living environment and warm service. It has free pick-up service. The hotel is located at the southeast side of the terminal building. You can dial 020-36067393 or 36062639 to book a room in advance.