http://www.stats.govt.nz/infoshare/SelectVariables.aspx?pxID=163d666b-55b3-476a-ba8b-d8507dd8b0e3

Flights are only counted if at least one individual on the flight is recorded as an arrival or a departure.
Although most flights are scheduled commercial services, the numbers may also include charter services, private aircraft, cargo aircraft and other special flights.
Flights may visit more than one overseas port, and will be counted under each. Hence, the sum across overseas ports may differ from the total number of flights.