One person has died after four people have been swept down the Waikato River, north of Taupo.
A staffer said the group were swimmers "in bikinis and togs", in their early 20s.
Mercury Energy is reviewing its procedures to see if it can improve safety after a woman was swept to her death by the surging waters of the Waikato River yesterday.
The woman, believed to be from Auckland and in her early 20s, was one of a group swimming in the river shortly before midday. Four were swept away when the Aratiatia Dam gates opened sending waters of up to 90,000 litres a second downstream.
Two women and a man managed to scramble to safety but the third woman's body was later found in one of the pools in the Waikato River.
One man told the Herald up to six people were in the water initially.
"Two women made it across to the bank on the far side of the footpath. One had a selfie stick and initially they didn't seem fazed by the situation," he said. "The man went back into the water after reaching the bank to help the other women."
A police spokeswoman said a larger group was in the water initially - but only four were swept away in the turbulent waters, while other friends raised the alarm.
Power authorities open the Aratiatia Dam floodgates upstream several times a day. In the summer, they are released at 10am, midday, 2pm and 4pm.
A siren sounds before the water is released and there are signs warning people of the danger and not to swim there.
Mercury Energy chief executive Fraser Whineray described yesterday's death as an "absolute tragedy" that has "devastated" staff.
"Our sincere thoughts go out to the family and friends of the person who has passed and to those who are recovering at the local hospital and who will be traumatised," he said.
A range of measures are in place to manage the hazard, Whineray said, including extensive signs by Mercury and the Department of Conservation.
There is also fencing in place and a siren sounds just before the flood gates opened.
Chopper over Waikato River where a swimmer drowned.
Whineray confirmed the siren was working yesterday.
"We've been running these spills for years; in fact they are a condition of our consent because it is part of the tourism environment there.
"But this is a very concerning event and we will be reviewing all of the many measures we've put in place."
Whineray said the company would be looking to see what else it could do to reduce the risk as much as possible to prevent future tragedies.
A Huka Falls River Cruise worker said when the waters surged the river was transformed from being "very calm" to intense rapids.
"'No swimming' signs are in place," she said. "These people obviously didn't read the signs saying don't swim beyond the rapids and went swimming.
"When the water was released at 12pm, the water came down and flooded the river and swept them away."
The release from the Aratiatia Dam transforms calmer waters into rapids.
Chief executive of Water Safety New Zealand, Jonty Mills, said yesterday's events were "tragic" for both the families and the community as a whole.
He said the Waikato River was "notoriously" dangerous - last year, nine men drowned in the area.
This figure was up 31 per cent on 2015 and was 55 per cent higher than the five-year average for the region.
Although he understood the Kiwi desire to get out on the water - he said people needed to be aware of the local conditions and activities that could affect swimming conditions.
"It's part of what we do as Kiwis, but we also want them to come home safely with their families, so they need to be prepared and know their own limits."
Monday's tragedy brings the water-related deaths this Waitangi Weekend to five, after four others ranging in age from 21 months to 70 years died in rivers, pools and at sea, on Sunday.
Early Sunday morning a diver collapsed and died after reaching the shore at Anaura Bay, north of Gisborne.
In the afternoon, a 21-month-old boy drowned in a swimming pool in Makarau, north of Auckland, a 30-year-old man died after being pulled from the Wellington Harbour and a 70-year-old woman died after being swept out to sea by a "freak wave" while walking with her partner along the Kohaihai River, in Kahurangi National Park.
Her body was found later that evening after an extensive search by sea and air.
Water-related deaths this weekend
Sunday 3:30pm: 70-year-old woman dies after being swept out to sea while walking with her partner along the Kohaihai River, in Kahurangi National Park.
Sunday 3.30pm: 21-month-old boy drowns in swimming pool in Makarau, north of Auckland.
