Whanganui is a city near the mouth of the Whanganui river on the south shore of New Zealand's North Island. New Zealand post has their "Collectibles and Solutions" center there, where they fulfill orders and carry out other philatelic services.
The postmark on this cover has an image of a steamship on it. At one time these were used on the Whanganui river for passenger and cargo transportation. The image on this postmark might be of the PS Waimarie: a restored paddle steamer that now offers tourist cruises on the Whanganui river.