"We sat there arguing for a little while whether he was seeing things or not.
"I told them to have a look near the rapids and sure enough, he saw the boy in the middle of the river, on the bottom, by the rapids," she said.
Ms Riwai called emergency services and a St John Ambulance crew headed to the scene. Daniel Watson hauled the boy from the river, she said, and his brother Thomas had to help with the retrieval because the strong current was dragging them towards the white water.
"Daniel had the boy in his arms and his brother had to keep pushing him along because there was no way he was going to get out by himself, " Ms Riwai said.
"That boy was dead when they got him out. He was completely blue, all of his body was blue, and he was flopped over in their arms. I told the other kids to stay by me because I thought he was dead.
"They did CPR and it took three goes to resuscitate him. Daniel pushed on his chest and Tom blew air into his lungs. I was on the phone to St John's when they brought him round. He was just breathing and shivering when the ambulance turned up about 20 minutes later but he wasn't responding to any talking or anything," she said.
The boy had been under the water "for at least five minutes", she said, and his rescuers were still dealing with the aftermath of the critical moments that had saved his life.
"He was dead when they finally got to him, and Daniel and Tom have both had nightmares about trying to pull the boy out but being a few seconds too late, and losing him to the rapids."
Ms Riwai said most of the group of children were pupils from Hillcrest School in Pahiatua.
She was surprised there was no adult supervision of the children who were swimming.
The boy was still recovering from his near-fatal ordeal and Ms Riwai and her partner were yet to speak to him.