谢谢ls上的！
这里也找到书面back up：
If the application is approved you will be issued three-year multiple entry visitor’s visa, authorising a permit for a visit of six months; from your date of arrival. You will not be able to extend the permit, however you will be able to enter and leave New Zealand during the validity your visa, as long as you do not remain New Zealand for more than 18 months during the validity of the visa.