A cost effective way is to take the Sky Bus from the airport. The bus is $18 one way and will drop you in the city centre. Tickets can be purchased in advance and presented to the driver on your smart phone, or at the kiosk outside the terminal.
Taxis can be taken from outside the arrival terminal. Costs vary. There are many companies, the following is a selection only.
Auckland Co-op Taxis 09 300 3000
Green Cabs 0800 464 7336
Discount Taxi 09 529 1000