I think the situation is that a boat may visit without paying duty as it is not imported. However that stay is limited.
If it is sold in NZ duty applies and that works out around 20% including gst.
I don't think there is any gst refund for non residents on departure.
Duty and tax would not apply if sold offshore
after clearance eg on transit to Fiji
if the vessel had not been imported.
Whether it would work to simply go 12 miles offshore having received clearance under one owner who then is not permitted to stop elsewhere in NZ and make a new entry under a new owner and then be subject to the same terms as a new arrival is debatable. Simply going out of territorial waters would not count as leaving NZ as some transits may do this simply on a coastal cruise
. Whether the change in ownership
on the high seas would be accepted in this way would require specialist rulings.
If the ownership
involved different nationalities and a change of registration
may be another complication i.e doing this in transit if required prior to arrival.