The 15-year-old Takapuna Grammar School student stepped in when he saw they weren't responding in the correct way.
Had he not done so, Conaghan's father Dave says, the girl would likely have died right there.
"We're very proud but it was an extremely stressful situation," he told the Herald.
As police, St John and the Auckland Westpac Rescue Helicopter rushed to the park, Conaghan got to work using the "DRSABCD" resuscitation method - standing for Danger, Response, Send for help, Airway, Breathing, CPR and Defibrillation.
There was no doubt in his mind the girl's life was on the line because her skin had turned blue and there was stuff coming out of her mouth, he said.
Alongside the two women, Conaghan gave the girl two breaths, 30 compressions twice and another two breaths and 16 compressions before she regained consciousness.
"It was my first time ever using [CPR]," Conaghan said.
"It was quite stressful."
He says the girl's mother was screaming.
"Hero Teen" Rocco Conaghan,15, was involved in the rescue. Photo / Supplied
The rescue helicopter arrived when CPR was being performed. Her condition at the time was critical, a spokesman of the crew said.
However, en route to Starship Children's Hospital her status improved to serious.
The girl's condition continued to improve, and an Auckland District Health Board spokeswoman said she was discharged yesterday.
Water Safety NZ figures show there were 69 preventable drowning deaths last year, 13 fewer than the year before.
