"There were people on the ground in various twisted states, with others wandering around in shock," Merrett said.
"It looked like they'd been travelling at speed and had hit the bank of a stream that runs down the beach. Two of the victims had been thrown upwards of 20m."
Merrett and Scott quickly realised that one of the teens on the ground had a spinal injury, while another had suffered a broken femur. A third had a severe laceration on his knee, concussion and a suspected fractured wrist.
"We had to put the broken femur in a splint with no pain relief – you can imagine what that was like for the victim."
Westpac helicopters were called in from Palmerston North and Wellington, but rising tides meant the closest landing point was 650m down the beach from where the accident took place.
Volunteer surf lifeguards prepared a landing site and stabilised the victims so they could be transported by stretcher to the waiting helicopters.
The teens were incredibly lucky that the training programme was taking place that day, otherwise it could have taken much longer for help to arrive, Merrett said.
Riversdale Surf Life Saving Club would have been "extremely stretched" to help out.
"It takes about six people to do a log roll, which is how you move people with back injuries on to a stretcher. Riversdale might have three people on call at a given time."
In total, 16 surf lifeguards were involved in the incident for an hour-and-a-half.
SLSNZ chief executive Paul Dalton said he was proud of the way staff and volunteers responded with efficiency and skill on the day.
"Our surf lifeguards don't have an 'off switch'. This is a perfect example of our volunteers jumping in and doing what needs to be done in order to keep Kiwis safe on our beaches – in and out of the water," he said.
