Prince William's forebears may once have boasted they ruled the waves, but it was his wife, Kate, who triumphed when the British royals took part in a yacht race in New Zealand on Friday.

Under the tutelage of expert sailors from Team New Zealand, the pair took to Auckland Harbour in rival America's Cup racing yachts for a maritime showdown that William joked beforehand contained "a bit of healthy competition".

Dozens of spectator boats weighed anchor nearby, some sporting signs that read "Team Kate" or "Team Will".

While the second in line to the throne had the edge in experience, having sailed with Team NZ before, it was Kate who claimed bragging rights, cruising to a 2-0 victory after winning both legs of their competition.

"We were sabotaged," William claimed after the defeat, later saying he only lost because "I wanted a quiet night".

Kate, nautically attired in a blue-and-white striped top, maintained a diplomatic silence but gave a thumbs up to the thousands of people who lined the shores of New Zealand's largest city to watch.

While they were on the water, a plane hired by lobby group Republic New Zealand flew overhead trailing a banner reading, "Time for a Kiwi head of state".

"Having the UK supply us with a head of state no longer works for New Zealand, we need our own head of state," the group said in a statement.

There were no such sentiments among the flag-waving crowds, who cheered as the royal couple stayed to shake hands despite the return of the rainy weather that has dogged the first five days of their 10-day New Zealand tour.

The royals also met New Zealand air force personnel at a base just outside Auckland and crossed Auckland Harbour on a "sealegs" amphibious vessel, which resembles a dinghy with legs attached to the bottom that can be deployed when it reaches land.

After they made it to shore, the manufacturers gave William a fully functioning mini version for baby Prince George, who is traveling with his parents on the three-week tour of New Zealand and Australia but who on Friday remained at their Wellington base.