Kate could not hide her delight when she beat her husband and was seen punching the air on both occasions her sleek racing yacht finished first with her at the helm.
Back on dry land, the Duke joked "we were sabotaged" and when someone said the Duchess was looking pleased he replied: "I bet she is. Selfless husband. I wanted a quiet night."
The Duchess' past experience on the sea no doubt counted as during her gap year she crewed on Round the World Challenge boats in the Solent.
The couple are said to have a competitive streak and the victories were revenge for the Duchess who lost to the Duke in a dragon boat race during their tour of Canada in 2011.
Eric Haagh, 36, the skipper of the Duke's boat, conceded the Duchess had won both of the two three-mile races but that their yacht had developed problems.
Mr Haagh said: "William was a bit nervous at the start. These boats are very powerful and he just wanted to get on the grinder (sail winch) to begin with.
"When we wiped out because of the problem with the sail we were near to some rocks and William said, 'That'll be a good photo'.
"William was a good sailor but Kate pushed us around into a bad position. She did very well.
"William was very competitive right from the start - he didn't want to lose. When Kate won both the races he just said, 'At least she'll be happy'."
William and Kate had asked for the opportunity to go sailing on Waitemata Harbour said Grant Dalton, managing director of Emirates Team New Zealand, which won consecutive victories in 1995 and 2000 in the world's premier yachting race the America's Cup.
But before they took to the water the couple enjoyed a private lunch on the 144ft super yacht of Swiss-Italian billionaire and Team New Zealand benefactor Matteo de Nora.
Once in the harbour they were pitted against each other on two America's Cup class 80ft sloops.
As they set off on their respective boats, the Duchess - who was wearing a striped Breton-style top by Me and Em and skinny navy jeans, patted William on the back and said: "Take care." He replied: "Yeah, see you later."
The skipper of Kate's boat was Dean Barker, Team New Zealand captain and a crew member of his nation's 2000 America's Cup team, who had taken William sailing in Auckland in 2010.
The Duke and Duchess wore baseball caps and lifejackets and were given the job of taking the helm.
The first race saw Kate quickly open up clear water between herself and William and she never looked like losing.
When the contest ended the boats passed close to each other and she smiled and punched the air and shouted something out to her husband.
The rematch quickly followed the pattern of the opening race with the Duchess' yacht streaking ahead, but William's crew could be seen battling with sail problems.
After the race the Duke and Duchess had a thrilling, bouncy ride across the harbour in an amphibious vehicle.
The royal couple grinned broadly as they took turns at the wheel of the Sealegs, which can reach speeds of 40 knots and then be driven like a car on land.
Afterwards they were given a miniature Sealegs custom-made for baby George.
David Glen, Sealegs chief executive, said: "It's an inflatable, amphibious little contraption for youngsters. It is absolutely bespoke. It was finished yesterday - four weeks from conception to completion."
He added that the Duke and Duchess were "astounded," he said: "They cannot wait for it to get to Kensington Palace. But they have to work out where to use it. The Duchess said she regretted he was not here to give it a try now."
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