A group of teenagers tried to scramble on to rocks as a flash flood swept through them, killing two of their group.

The five boys, all from Auckland and aged 17, had been swimming at the Cascade Falls area in the Waitakere Ranges when heavy rain struck around 4.15pm yesterday.

They had tried to scramble onto rocks before attempting to cross to the other side to reach their vehicle. Two had made it to the river bank but three were swept away, police confirmed today.

One then ran to a nearby golf course to raise the alarm. He was taken to Waitakere Hospital with mild hypothermia.

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One of the trio swept away managed to grab hold of a tree on the bank of the stream, and was eventually winched to safety by the Westpac Rescue Helicopter about 7:15pm.

Two of the boys were found deceased, Inspector Fata Willi Fanene said. They have been identified as Sosi Turagaiviu and Mitch Woolley, both 17.

At about 6.40pm Sosi Turagaiviu, of Massey, was found in the water under a bridge on Bethells Road, near Te Henga Road, Fanene said.

Then, at about 7:50pm, the body of Mitch Woolley, from Waitakere, was located by search and rescue teams in the Cascade Falls area near where they were swimming.

The boys' deaths have been referred to the coroner.

Fanene said the boys' families were "very distraught".

"We have been talking to the families of the victims of this tragic incident, and we are ensuring they are supported through what is an incredibly trying time.

"We are also making sure the three other young men who were caught up in this event are looked after, as this has been a very traumatic experience for them and their families.

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"An incident like this is an awful reminder of the power and unpredictability of our natural environment."

It was not clear if the boys had been aware of the impending storm that struck the area yesterday, Fanene said. It was concerning that people had been out swimming when there had been warnings about sudden rainfall.

The normally tranquil stream from Cascade Falls became like Huka Falls as heavy rain struck the Waitakere Ranges. Photo / Supplied
The normally tranquil stream from Cascade Falls became like Huka Falls as heavy rain struck the Waitakere Ranges. Photo / Supplied

The Auckland Westpac Rescue Helicopter was called at 5.30pm to help with the search and rescue operation.

Pilot Rob Arrowsmith said he, crew member Ati Wynyard and intensive care paramedic Russell "Rusty" Clark initially believed they were rescuing two people who had been swept away - but on speaking with them realised there were another three missing.

The normally gentle Cascade Falls had swollen to look like "Huka Falls in a picturesque bushy valley", he said.

He flew downstream as far as Bethells Rd, where one person was found dead.

Concerned the body might be washed further downstream, Arrowsmith flew to pick up firefighters nearby and dropped them off to look after the dead person.

Flying back upstream to continue the search, the water had receded by several metres within half an hour, he said.

"It was extremely hard to search underneath the underhanging foliage on either side of the river. We were lucky in glancing down, there was a guy in boardshorts underneath a tree.

"He was spotted through a gap in the trees. We winched Rusty down to him. This guy was in waist-deep water, with the main flow to one side. He was up against a tree with water between him and a bank, cutting him off."

The young man was winched up to the helicopter and taken to a waiting ambulance.

"He had a lot of abrasions and wounds on his back. But considering the flow, he was lucky he had no broken bones or deep lacerations. There were logs and rocks tumbling down the river bed and he would have been amongst them," Arrowsmith said.

Searching continued till dark, with police search and rescue eventually finding the body of a second person.

Arrowsmith said the encounter was "very sad and sobering".

He understood the five may not have seen the warning clouds before the flash flooding due to the direction of the weather system, which formed further south.

"By the grace of God, we've all been in a situation like that. I'd had to imagine what it was like with a lot of water in there."

The crew had worked "extremely hard" during the search, he said.

"Ati and Rusty spent two hours hanging out of the helicopter in pouring rain - it takes a lot of concentration and intention to keep watching for that time. And then Rusty was winched down into a raging torrent."

Arrowsmith said the crew were "incredibly sad for their families the whole time".

"It's just such a normal thing to be doing in New Zealand - taking a regular trip to a waterhole...If they were out mountaineering or canyoning you can understand it but we couldn't believe it could happen in such a regular, easy spot that so many people use."