Chilling moment four women are stranded on a rock in the middle of a river as a dam's flood gates open - before one is washed away and drowned

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A 21-year-old woman has drowned after floodgates were opened, sending water surging down the river where she was swimming with friends.

Three other swimmers made it to the shore of Waikato River, on New Zealand's North Island, but Auckland student Rachael Louise De Jong tragically did not survive the sudden onslaught of water.  

Her body was recovered from a rockpool by police on Monday night, stuff.co.nz reported.

Two German tourists, Katrin Taylor and Kevin Kiau, were on a viewing platform in the Aratiatia Dam area and could see four people standing on a submerged rock

Rachael Louise De Jong, 21, drowned after the Aratiatia Dam floodgates were opened, sending water surging down the spillway where she was swimming with friends

Two German tourists, Katrin Taylor and Kevin Kiau, were on a viewing platform in the Aratiatia Dam area and could see four people standing on a submerged rock.

They had heard five minute warning siren sound before the floodgates at the dam are opened. 

As the waters began rising, the two tourists watched as the girls tried to jump to a nearby rock where a man was standing.

'We saw the first girl made it. The guy pulled her in. The second girl jumped and made it safe as well – the guy pulled her in,' Ms Taylor told stuff.co.nz.

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

Ms Taylor said the man and the third woman were washed away.

'There was still one girl left in the middle of the river. I'm not sure if she tried to jump, or the water was already too high, but she was gone pretty quick without making it to the safer rock, to the larger one,' she said.

Two women were left stranded on the rock, left to fear the fate of their friends. 

Touching tributes from devastated friends and family describe a constantly smiling, bubbly woman who was loved by many

'You can live a lifetime and never meet another person that has such a profoundly positive effect in your life,' just one of the dozens of hearfelt tributes for the 21-year-old

Tragically, 21-year-old Rachael Louise De Jong did not survive the accident.

Touching tributes from friends and family describe a constantly smiling, bubbly woman who was loved by many.

'I am honestly lost for words hearing yesterday that we have lost you. The most beautiful girl inside and out I have ever met, just gone like that,' one friend wrote.

Another added: 'It is so rare that you meet someone so Beautiful inside and out like Rachael. You can live a lifetime and never meet another person that has such a profoundly positive effect in your life.'

Ms De Jong's heartbroken brother also expressed his grief over the  loss of his beloved sister.

'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,' he said.

'You never spoke a bad word of anyone, and you had such an infectious smile that could cheer anyone up. There's not enough words in the world I could use to describe you. I love you so much Rachael, rest easy.' 

The Aratiatia Rapids pictured when the spillway is open. As the waters began rising, the two tourists watched as the girls tried to jump to a nearby rock where a man was standing

Mercury Energy, the company that operate the spillway, confirmed there was a warning siren before the dam's gates were opened at midday

The Vice Chancellor of the Auckland University of Technology where Ms De Jong studied expressed sadness over the young woman's passing.

'Rachael De Jong was a student in our Physiotherapy school and was known as a bright, cheerful, kind and diligent member of the student community and was well respected by her lecturers and peers,' Vice Chancellor Derek McCormack said. 

'Our thoughts are with Rachael's family and friends at this extremely difficult and sad time. AUT extends our sincere condolences.'  

Mercury Energy, the company that operate the spillway, confirmed there was a warning siren before the dam's gates were opened at midday.

During summer, the gates open four times a day at 10am, 12pm, 2pm and 4pm, Stuff reports. 

'Mercury’s thoughts and condolences are with the De Jong family during this difficult time, they told Daily Mail Australia. 

'The drowning is a real tragedy and Mercury is working with relevant organisation’s to review safety processes in the area.'

'The spillway is opened approximately 1,200 times a year, for operational management on the Waikato River and for tourism viewing purposes.' 

There are signs around the area warning people not to swim as well and fences and harsh terrain to prevent swimmers from accessing various areas. 

'There are safety mitigations warning of the dangers of entering the active spillway area and a bylaw prohibits people from being in the spillway and downstream vicinity.' 

Ms De Jong's death has been referred to the Coroner.