A student drowned in a river just moments after posing for selfies when a dam was opened and the water level rose dramatically.

New Zealander Rachael Louise De Jong was swept away by rapids after becoming stranded on a rock with three friends in the middle of the Waikato River.

The four women had reportedly been taking selfies moments before, as the warning siren went off, signalling the dam would be opened.

The group attempted to jump to safety from the small rock on to a bigger rock as fast-flowing water began to rise around them.

Two women reached the rock, but Ms De Jong, 21, another woman, and a man who had been trying to help them were swept away by a sudden surge of water.

While the man and other women managed to swim to safety, Ms De Jong was drowned by the powerful current.

Rachael De Jong, a 21-year-old Auckland University student, drowned (Facebook/ Rachael De Jong)

German tourists Katrin Taylor and Kevin Kiau witnessed the incident, but were unable to help.

They told news site stuff.nz they were on a viewing platform when the siren went off and the water began to rise.

Ms Taylor said the water had become “quite high” when they noticed four people “holding selfie sticks” on the other side of the river.

“We could see the water was rising further and that they were in danger of getting washed away,” she said.

Ms Tayor said the women tried to jump to a bigger rock where a man was waiting whilst the water rose above their feet.

“We saw the first girl made it. The guy pulled her in. The second girl jumped and made it safe as well.

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

She said that they then saw the man and two of the girls being washed away – one of whom was Ms De Jong whose body was recovered by police later in a rockpool.

"There was nothing we could have done. We could just stand there and watch helplessly and it was horrible,” Ms Taylor added.

Ms De Jong, a student at Auckland University, was described as the “most perfect sister” in a tribute posted online by her brother, Daniel.

“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.

”You never spoke a bad word of anyone, and you had such an infectious smile that could cheer anyone up. I love you so much Rachael, rest easy.“

The Aratiatia Dam, operated by energy company Mercury Energy, is opened four times each day during the summer. As well as the warning siren, there are signs around the river telling people not to swim.

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CEO of Mercury Energy Fraser Whineray said in a statement that safety processes would be reviewed in light of the incident.

”We are always looking for ways to improve safety to minimise the risks of it ever happening again,“ he said. ”The challenge is to keep people out of that area at those times [when the water is released].