Hi Sally
I have information obtained from the National Archives, that the 2nd Battalion, 18th Regiment left NZ in February 1870. But like all information regarding a regiment's 'station' and movements this usually relates to the commanding officer. Being overseas they would have fewer ships to call upon, so it would not surprise me if they were actually transported over the course of a number of months.
A book was published about 20 years ago on men taking their discharge in NZ, I would ask on the NZ board in the hope that someone has a copy.
Ken