A group of swimmers cling to a rock in the middle of the Aratiatia Rapids in Waikato River moments before three of them were swept away.

All they could do was watch.

Two German tourists were standing on a viewing platform halfway down the Aratiatia Rapids to witness a calm river turn into a raging torrent.

Instead they watched as three swimmers were swept away, one to her death.

FACEBOOK Rachael Louise De Jong, 21, was swimming with friends in the Aratiatia Dam area of the Waikato River when she got into trouble and drowned.

German tourists Katrin Taylor and Kevin Kiau were on the platform, north of Taupo, at noon on Waitangi Day ready for Mercury Energy to open the dam floodgates upstream.

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They heard the five minute warning siren sound.

Supplied The Aratiatia Rapids photographed at 12 noon on Monday, and then five minutes later after the spillway was opened.

Then the Waikato River started to rise.

When the water was "quite high" they looked to the other side of the river and saw four people standing in the rapids on a submerged rock.

They were holding selfie sticks, Taylor said.

STEPH RANGI/FAIRFAX NZ The calm pool below the Aratiatia Dam where Rachael Louise De Jong was swept to her death.

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

Taylor said the girls tried to jump across to a bigger 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."

The water level kept rising until it was above the feet of the two girls left on the rock, Taylor said.

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

The man was washed into the river still holding her, Taylor said.

"We couldn't see what happened after that because the rock was in the way and we had no clear view.

"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.

"So three people were washed away by the water and the other two girls were standing there [on the larger rock]. They were trapped because the water was left and right of the [big] rock – they couldn't go anywhere."

Taylor said she didn't realise at the time that someone died in the accident and only found out on Tuesday night.

"We thought they would have broken bones but they would come out alive. There was nothing we could have done. We could just stand there and watch helplessly and it was horrible."

Someone near them called 111.

Later, police recovered the body of Auckland University student Rachael Louise De Jong, 21, from a rockpool downstream.

De Jong's death was one of seven in five days as sweltering temperatures and clear skies saw holidaymakers swarm to popular swimming spots over Waitangi weekend.

Two women, four men and a 21-month-old boy died in water-related incidents across the country.

Just over a month into the year, New Zealand's annual water death toll has already reached 18.

Water Safety New Zealand chief executive Jonty Mills said greater vigilance was needed around rivers.

He said personal responsibility and taking care of others could help.

"People need to know the local conditions, and understand local conditions. Rivers are certainly an area of concern. They made up nearly 30 per cent of preventable drownings."

The Waikato River in particular is notoriously dangerous.

In 2016 there were 17 preventable drownings in the Waikato and nine of those were in rivers.

"That's up significantly on the previous year and the five year average." He urged people to check before jumping into rivers, know the risks and never swim alone.

Mercury Energy CEO Fraser Whineray said safety processes are being reviewed in light of the Aratiatia Dam tragedy.

"We are always looking for ways to improve safety to minimise the risks of it ever happening again," he said.

The spillway is opened more than 1200 times a year at posted times and is a popular tourist attraction.

There are signs posted throughout the area warning about the dangers of entering the rapids. Sirens blare at intervals before each release of water. Fences mark off safe areas and difficult terrain also forms a natural barrier.

Whineray said the topography of the rapids makes camera solutions impossible to rely on.

"The rapids are quite long and have corners making digital solutions for that quite a challenge," he said.

"The challenge is to keep people out of that area at those times [when the water is released]."

Taupo Mayor David Trewavas said Mercury Energy works with council to review safety processes for the spillway.

"There are signs up, sirens going off so it is a case of what else can you do?" he said.

"We'll see what the review brings back and if there are any recommendations we can look at those and do what is required."