Team New Zealand's two boats were side by side nearing the end of a practice race when the mast on NZL57 snapped.

"We heard a loud bang and not long after that the rig started to break. It fell straight down into the water," Team New Zealand skipper Dean Barker said.

"It's obviously very disappointing. It has a big impact on our programme," Barker told Radio Sport.

He said the break was due to a mechanical or structural fault, but the cause could not be found until the boat was returned to shore.

Fortunate the team have a spare rig and will be back on the water with both boats by Monday.