What goes for the animals evidently also is valid for the plants. The flora got isolated from the evolution in the rest of the world, evolved on its own and came up with its own forms and species. I don't know which species of plants are endemic on Aotearoa. Later on, when the Maori and later the Europeans arrived, they brought their own plants, and finally, at the present day, New Zealand is also part of the international exchange of plants, e.g. for gardens, where many people like to have exotic plants or man-made varieties with big flowers and extravagant colours. The government doesn't allow all kinds of seed and plants to enter the country, in order to preserve the endemic flora (and fauna) and to protect them from invasive introduced species that can take over whole swatches of land and squeeze out the natives.