A young woman has died after she was swept away when the floodgates opened at a New Zealand river she was swimming at with friends.

University student Rachael De Jong was visiting the Waikato River with three friends on Monday where they were seen taking photos together in the water, The Sun reports .

The group were on a rock in the rapids with a selfie stick when the five-minute warning siren for the floodgates sounded.

Three of the women made it to a safer rock before the floodgates opened, sending water surging down the river, but Ms De Jong, 21, was swept away.

Her body was later recovered from a rock pool by emergency services.

German tourists Katrin Taylor and Kevin Kiau told stuff.co.nz they saw the event unfold from a nearby viewing platform.

"They were holding selfie sticks," Ms Taylor said.

"We could see the water was rising further and that they were in danger of getting washed away."

Ms Taylor said she saw a man manage to pull two of the girls to safety on a rock.

"We saw the first girl made it. The guy pulled her in. The second girl jumped and made it safe as well," she said.

"The third girl, she jumped but the water was washing her away so the guy grabbed her."

The pair said they were sure the man got washed away trying to help the girls, but must have later made it to safety.

Ms De Long's brother Daniel wrote a touching tribute to his sister on Facebook following the tragic incident.

"Yesterday I lost one of the most important people in my life, my wonderful sister," he wrote.

"Not only was she an inspiration to us all, she was my best friend, and the most perfect sister I could ever have asked for.

"I can't even begin to describe how much I'm going to miss you, and how incredibly unfair it is that you have been taken far, far too soon.

"You never spoke a bad word of anyone, and you had such an infectious smile that could cheer anyone up."

The photo taken moments before the floodgates opened.