Ms Kerr said she and her colleagues instinctively launched into action.
"I saw the man on the ground and it was clear he was in trouble. I got down on the ground to check for vital signs and basically switched into what I'd been taught as part of my crew training.
"I tried to see if he was breathing by watching his chest and listening for breathing sounds. But it was really hard to tell because he was wearing a thick jersey and it was windy," she said.
"I did what I'd been taught ...tapping him on the chest and shouting at him to see if I could get a response."
The group administered CPR as they waited for an ambulance and the airport's medical staff to arrive.
They also helped to calm the man's wife and get the children away from the distressing scene.
Ms Kerr said: "He made a noise and lifted a hand just as the paramedics arrived.
"I was relieved to know that he was alive, and rolled him into the recovery position as the medical experts took over."
The flight crew later took the man's granddaughters - aged about 4 and 6 - to their Mangere home as their grandmother accompanied their grandfather to hospital.
"They were okay," Ms Kerr said.
"They were chatting to my colleague and me on the drive home, but naturally they were worried about their poppa."
A St John Ambulance spokeswoman said the man had a medical condition and was taken to Middlemore Hospital in a serious condition. It is understood he was admitted to intensive care.
Constable Paul Conyers of the airport police praised the three cabin crew members for their quick action, which he said had contributed greatly to the man's survival.
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