By law, all trains and platforms are designated non-smoking services (Smokefree Places Act 1990). This means that smoking is not permitted in the outdoor viewing carriage, the vestibules (areas between carriages), the toilets or any other area of the train.
However, we do ordinarily have scheduled stops en-route where there is sufficient time to step off the train and "stretch your legs". These are not designated breaks, so be sure to check with the onboard staff before disembarking.
Please be very careful when disposing of cigarette butts in New Zealand. Rural areas are prone to bushfires, especially during the summer, and cigarette butts can easily cause devastation.