Além da resposta do mts em relação às regras da companhia aérea para viajar pela Nova Zelândia.
Do site oficial da Air New Zealand :
Entrando na Nova Zelândia em um bilhete só de ida
People travelling to New Zealand on a one-way ticket must be one of
the following:
a New Zealand passport holder
an Australian passport holder a holder of a current permanent resident visa of Australia or
a holder of a current Australian Resident Return Visa allowing an indefinite stay
other foreign national holding a valid New Zealand Resident Visa, New Zealand Permanent Resident Visa, or New Zealand Citizen
Endorsement
-
a holder of a New Zealand visa that clearly states “Return/onward ticket not required”
All other visitors to New Zealand must possess a return or onward
ticket to a country to which you have right of entry. If you are a
visitor to New Zealand making a separate international journey from
and returning to New Zealand before departing New Zealand on your
onward ticket (e.g. travelling to Fiji and then returning to New
Zealand), please remember to take your onward ticket with you. You
will need to present this when checking in for your return flight to
New Zealand.
If you do not hold an appropriate passport, visa or return ticket you
may be required to purchase a ticket out of New Zealand to a country
that you have right of entry prior to you being able to travel. Such
tickets are non-refundable until you are fully able to satisfy the
entry requirements of New Zealand.