I don't intend to be discouraging but you might have trouble finding many boats heading to New Zealand
in August or September. A high percentage of cruising sailboats who have a destination
of NZ in mind will have been sailing westwards across the South Pacific
and will likely depart from Tonga
in the late October/early November time frame.
It's altogether possible that when we were preparing to leave Nuku'alofa in Tonga
for New Zealand, if we had been approached by someone exuding charm and didn't seem too scary, we might have offered them a berth on board our boat
. Lucky for us our passage
was delightful but we were prepared for the worst as it can be a very uncomfortable 1,200 miles. However, I would caution anyone of traveling to Tonga in the hopes of catching a ride to NZ - the Tongan Immigration can be highly suspicious of people arriving on a one-way ticket.
On the other hand, our 25-year old daughter who was traveling with us, found excellent surf in NZ on long uncrowded beach breaks and she even became a certified Surf Lifeguard.
Good luck, fair winds and calm seas.