Monarch Butterflies are not native to New Zealand. They are thought to have originated in central America, and they spread across the pacific in the nineteenth century:
The exact date Monarchs first arrived in New Zealand is debated. However their caterpillars can only survive on certain species of the Asclepiadoideae or “milkweeds” subfamily. None of these hostplants for Monarch caterpillars are native to New Zealand, which means that any Monarch butterflies arriving in New Zealand would have been unable to reproduce until Europeans bought in the now well-known “Swan plant” (Gomphocarpus fruticosus, also formerly known as Asclepias fruticosa) and other host plants.
So the New Zealand population of Monarchs are dependant upon people growing milkweeds in their gardens. If introduced milkweeds disappear so will our beloved Monarchs.