Four women stranded on a rock in the middle of a river as a dam flood gates opened, one got washed away and drowned

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, 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.

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

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. 

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 will be a safety review with Mercury Energy and the Taupo council to safety processes for the spillway. 

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

Daily Mail Australia has contacted Mercury Energy for comment.