Hi,
I think the ferry you are meaning is the one to Devonport.
Heres a link to the timetable.
http://www.fullers.co.nz/index.php/pi_pageid/18

Leaves from the ferry buildings downtown.Heres a shot from when Regal Princess was in town.
[Linked Image]
The ferry building is the old brick one.It is only about 12 minutes across the harbour.Devonport is a delightful old village with galleries and cafes.It gives great views back to the city.

David.