Emirates Team New Zealand have capsized in a dramatic America's Cup qualifying series semi-final against Britain's BAR in Bermuda.

British skipper Ben Ainslie's crew had won the start in strong winds on the Great Sound when the Kiwi boat rode too high on its foils and nose-dived into the rough seas.

Ben Ainslie's boat stopped immediately and went to help.

"I mean, thank god it sounds like everyone's okay," Ainslie said. "That's the most important thing. What an incredible day of racing. 30 years of racing sailing boats and I've never experienced anything quite like that."

Reporter Dana Johannsen is in Bermuda and told Mike Hosking it was terrifying to watch.

"There's serious damage to both their wing and their platform. I think they're in serious trouble if they're going to be able to get out on the water tomorrow, that's for sure."

Helmsman Peter Burling spoke at a press conference after the catastrophic capsize.

"Right now the guys have obviously taken the boat out and assessing the damage, which we have quite a bit, but we feel like well be able to repair it and get back into action," he said.

"I think we've all got a few bruises and cuts and bits and pieces, but nothing major and I was very thankful when I was sitting up the top to be able to look out the back and see all the heads above water and know they're all safe."

Mel Ashby, wife of skipper Glenn Ashby, said it came as a shock.

"Initially it took a minute to process. It happened so quickly."

All crew were accounted for and unhurt, and the boat righted after nine minutes, although there looked to be extensive damage.

Spectacular capsize by @EmiratesTeamNZ #allsafe
Drama during race 4 of the Louis Vuitton America's Cup Challenger Playoff vs @LandRoverBAR pic.twitter.com/MVvFcZaXAx

— America's Cup (@americascup) June 6, 2017

Sailing commentator Tony Rae said: "There looks like a lot of bits and pieces. It looks like a train crash at the moment but a lot of these bits are just very superficial. It's a bit like a Formula 1 car, you see bits flying off everywhere."

Team NZ lead the best-of-nine America's Cup semi-final qualifying series 3-1, after adding a win in the first race of the day to the two from Tuesday (NZT).

Ainslie's crew led for half of the race but were overtaken by Team New Zealand after losing ground on a crucial tack.

The New Zealand boat managed to lift on its foils for more of the course in the choppy waters, reaching speeds of more than 40 knots at times in the high winds.

Analyst Chris Steele said aside from the drama, the lost race and damage to the boat, Team New Zealand's secrets will also have been revealed.

"With the boat on it's side like this you're revealing everything that's underneath the boat, so these pictures and video that we're seeing will be recorded by Oracle and they'll be looking at the shapes of the foils, the rudders to get an idea of what's under water for Team New Zealand which they normally would not be able to see."

However Steel said they were already operating with their second wing sail today after replacing it earlier on.