An airport shuttle bus is available at the airport and buses are familiar with the location of all lodgings. Depending on your arrival time, it can take around 20 minutes from airport to city. The cost is around $20 – you pay in cash to the driver.
You can either catch a shuttle bus from outside the airport or make an online booking. Here are 2 options:
https://www.supershuttle.co.nz/Booking/
https://www.combinedshuttles.co.nz