The update Xiaotung is referring to is
here and is effective for travel ticketed on or after 24 July 2012, and says
Effective 24 July 2012 Air New Zealand’s infant travel ticketing policy will change - from this date if an infant turns 2 years of age at any point of the ticketed journey the infant must occupy a seat and purchase a child airfare where child fares are offered or an adult fare where child discounts are not offered..
The original policy can be found at
this link and says that Air NZ's policy was that 'Infants are defined as children under the age of 2 at date of
departure.'
However, they also say that for 'interline' fares (ie fares featuring another airline as well as Air NZ), "if an infant turns 2 years old during the journey the child fare level must be charged for the entire journey."
So I would assume that providing your travel was ticketed before 24 July and Air NZ is the only carrier on the ticket,there will be no problem. If the answer to either of these points is a 'no', the travel agent appears to have booked it incorrectly (Note: I'm not a travel or reservations agent so can't guarantee my interpretation is correct.)