Radio New Zealand said the 328m Sky Tower was evacuated on Saturday after threats were made by the pilot, who said the plane had been stolen from a local airport south of Auckland.
The tower sits on top of a large casino and hotel complex in the centre of Auckland, New Zealand's biggest city.
It was not immediately clear how many people were aboard the plane and what their condition was. Police boats were moving towards the spot where the plane went down.
Police were unavailable for comment and there were no further details.
The leaders of the two main political parties – Labour Prime Minister Helen Clark and National's Don Brash - were in the city awaiting the outcome of the general election.