Corey's twin, Callum, told the Weekend Herald that his brother had attempted a back-flip off a high bar and into a foam pit below.
It wasn't clear exactly how the injury happened.
Corey's friend Oscar Arnold reacted quickly - something Callum believes saved his brother's life.
Oscar said last night he and friend Ben Padden waited about 10 seconds, believing their friend would emerge.
"Ben heard him say, 'get me out of here'. Then we jumped in and tried to clear the foam," said Oscar.
"Ben created an air pocket for him to breathe, and I just talked to him and tried to calm him down. He was so scared."
Oscar said the accident had replayed in his head since: "I can still hear his voice ... his pleas for help. But he was so strong. It is amazing how well he is taking it all. It must be so terrifying for him.
"I was really moved. The first thing he said to his mum was that he loved her. It just made me realise that you've got to tell the people you love that you love them, because you never know how much can change."
Since the accident Corey's parents, Rochelle and Paul, have spent most of their time at Middlemore.
The family have been told Corey could remain a quadriplegic.
Callum, his siblings - 16-year-old twins Aimie and Bayden and Madeline, 5 - regularly visit, although Bayden is wary of hospitals after his own cancer ordeal four years ago.
Said Callum: "He is better now but he hates hospitals because of that. Because we have been through a traumatic experience like this before, we are handling it better than most families. But obviously still not that great. Everyone is being strong for Corey."
The family took heart from the early progress of wakeboarder Brad Smeele, left quadriplegic after a training accident in the United States in July.
Corey is unable to talk after a tracheotomy, but can mouth words and is keeping his spirits up.
"He is still himself - still the funniest kid on the block, making jokes and stuff. He got Best Sense of Humour in the year book at school. That's who he is," said Callum.
"He is a brave, tough little kid. He's a trooper. If anyone was going to make it through, he would. So fingers are crossed there."
Callum is keeping busy with Facebook and Givealittle pages he set up to give updates on his brother's progress, and let family and friends know how they can help.
Albany Senior High School is also rallying to help the family.
Corey's mum, Rochelle, told the Weekend Herald she was hugely grateful to the many people who had been in touch or offered support.
Oscar Paulich, chief executive of the trust that runs the North Harbour Gymnastics Centre, said Corey was participating in an adults gymnastics class that was unstructured.
Interviews with those in the gym at the time had established that Corey entered the foam pit head-first.
He had attempted a trick from a high bar, about 2m to 3m above. Minor injuries occurred in the pit very occasionally, but never one so severe.
Family look to wakeboarder as example
The family of Corey Sands say they have looked to wakeboarder Brad Smeele as an example of how small signs of recovery can be made.
Last night Mr Smeele, who was left quadriplegic after a training accident in July, gave a message of support to Corey.
It was important to stay positive despite the enormity of your injuries, Mr Smeele said, and maintain a determined "tough and strong" attitude.
That meant not listening to those who were negative about the possibility of recovery - even if they were doctors, who "know a lot, but not everything".
His own progress has included getting a small wiggle out of his right thumb.
"You have to look at each of those small goals as an individual win."
To donate visit: http://www.givealittle.co.nz/cause/Coreysands
