Immediately recognising she was in danger, he swam down, grabbed her by the foot and pulled her until he could touch the floor and carry her out.
"I came out of the changing room after getting dressed and walked around the corner to see Riley coming out of the pool holding Ella flopped in his arms, yelling to Dad for help," said Mel Drummond, Riley's mother.
"I had told Pete that I would go get changed then get the kids changed one by one.
"Riley had made a friend and asked if he could say goodbye to him. We think Ella just followed him in. Pete presumed I had her, and I presumed Pete had her," said Drummond, who together with her husband, found themselves in every parent's worst nightmare.
She said Pete grabbed Ella and she was completely blue and lifeless.
"I just stood there and screamed. Pete started doing CPR and my screaming alerted everyone in the pool area that something was wrong."
Off-duty nurses who happened to be at the complex raced over and helped to perform CPR.
After a few rounds of CPR, Ella regained consciousness.
"She vomited a whole lot of water but still hadn't started breathing. Another round of CPR, more vomiting and then she started screaming. It was very, very close," Drummond said.
Shortly after, St John ambulance officers and the local rescue helicopter service arrived and Ella was airlifted to hospital where she was kept overnight for observation.
The 5-year-old has Williams syndrome and endured four heart surgeries in the first year of her life. She has fully recovered from the near-drowning experience.
Riley was presented with an ASB Super Saver Bravery Award by St John Paramedic Tony Cronin and ASB school account manager Anna O'Hagen during an assembly at Waikawa Bay School on Friday.
The award – a surprise for the year three student – came with a golden superhero cape and certificate.
"Our little man is an absolute hero. He's saved his sister's life; we couldn't be prouder of him," Pete Drummond said.
As a self-described helicopter parent, Mel Drummond said the family has always kept a watchful eye on Ella, but since the incident, Riley has been extra protective of his sister and does not like letting her out of his sight.
"He's pretty traumatised by it to be perfectly honest. He's quite emotionally strung at the moment, but he's spent time with his grandparents, and we've sought professional help"
"He needs to know he saved her life. He kept asking things like, 'What if I wasn't there Mum,' and 'Dad, did I do a good thing? Is that what you would've done Dad?'"
A month on, Riley is now coming to understand his lifesaving actions.
"He said to us the other day, 'So Mum, what I did for Ella, does that wipe out any of the naughty things I've done in the past?'"
St John Head of Community Education, Jacci Tatnell said the 7-year-old is a pure example of how young children can save lives.
"Riley's lifesaving actions affirm the importance of teaching basic first aid to every child in New Zealand. That is why we are committed to delivering our ASB St John in Schools programme to tamariki throughout Aotearoa.
"We know that equipping children with the skills and confidence to take action in an emergency will save lives and build stronger, more resilient communities," said Tatnell.
She said each month the ambulance communications centres can receive as many as 50 emergency calls from children in traumatic circumstances where a loved one has fallen or is unconscious and there are no other adults in the house.
To date, more than 800,000 students have completed the ASB St John in Schools programme since 2015.
With support from ACC, the goal is to deliver it to a total of one million New Zealand students (pre-school through to intermediate) by 2023.