A couple who travelled to New Zealand to be married almost missed out on their dream day after the bride-to-be almost drowned during a dive trip.

Sophie and Gareth, who did not want their surname published, arrived last month from the United Kingdom ahead of their wedding day on Friday, January 29.

On Monday, the week of their wedding, friends organised a diving expedition to Great Mercury Island.

Cave Cruzer Adventures owner Steve Miller hosted the couple alongside his brother Richard, a divemaster, and Richard's partner, Jo, who know the couple.

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Mr Miller said he and Jo stayed on the vessel, while his brother took Sophie and Gareth below water.

Suddenly, they spotted Sophie resurfacing. Mr Miller said he could see she was in trouble, so he tossed a life-ring towards her. She disappeared beneath the surface, however, and the alarm was raised.

Richard Miller raced towards the seabed -- where he found her about 18m below surface.

"There was no time to p*** around ... she needed oxygen straight away. He just brought her up as quick as he could and got her on the boat.

"We got her on the boat and got stuck into the CPR because she was non-responsive and not breathing."

There was also a lot of blood in and under the apparatus on her nose and mouth -- something they put down to her being brought up to the surface at such a high speed.

After about five minutes of CPR on Sophie, she came to just as the Auckland Rescue Helicopter arrived at the scene.

She spent a couple of days at Auckland City Hospital, before being discharged in time for their wedding ceremony on Friday.

"It was just a small ceremony," Mr Miller said.

"They got married at a beach in Mt Maunganui. It was all smiles and happiness and a lovely day, from all accounts.

"It was one of those [stories] that worked out in the end. It could've gone completely pear-shaped, otherwise."

Mr Miller said this had been the third sea rescue he had been involved in. In the first one, the person survived. The second -- a friend -- sadly did not make it.

"I was thinking: 'Nah, this isn't happening again'. We just got on with what we've been trained to do and we were extremely lucky the sea decided to give us one back."

The couple, who did not want to speak to media, are travelling the country for the next few weeks for their honeymoon.