Fewer than one in six people survived cardiac arrests that happened outside hospitals last year, the latest figures from St John show.
If no ambulance officer witnessed the cardiac arrest, the odds of survival dropped to one in nine.
That's partly because only 60 per cent of victims received CPR - cardiopulmonary resuscitation - from a non-medic bystander. Being in a public place improves survival rates.
For every minute without CPR or defibrillation, a patient's chance of survival falls by 10-15 per cent, according to St John. The odds of survival double if a defibrillator or AED is used to shock the patient's heart.
Alec Smith was one of the lucky ones. The 57-year-old suffered a cardiac arrest at the New World supermarket in Bishopdale, Christchurch, on September 27 last year.
Kim McCormack was with her son Oliver as the "woozy-looking" Smith came toward them slumped over his trolley. A piece of cholesterol from a clogged artery had broken away and lodged in his heart, stopping blood flow.
McCormack, 42, caught Smith as he slid to the floor and put him in the recovery position while Oliver called 111 and got help.
Two New World staff set up a defibrillator as Smith stopped breathing and turned grey.
After the first shock, no heartbeat was detected and they started CPR - McCormack on chest compressions, a staff member trying to help him breathe.
McCormack kept pumping away for "quite a while" until the defibrillator detected a heartbeat. Paramedics arrived and took Smith away; that night they phoned to tell her he would pull through.
The former teacher had CPR training and stayed relatively calm as she tried to get Smith's heart pumping again.
She recommends everyone get CPR training - but if someone needs help, "don't worry about doing it wrong, just go and do it", she said.
Smith now has stents and feels "like an 18-year-old again" - his wife Diane struggles to keep up.
Both feel very lucky McCormack knew CPR. "If it had been anyone other than Kim they might not have known what to do," he said.
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It takes seven minutes on average for an ambulance to arrive after being called.
Intensive care paramedic Curt Ward, one of the first ambulance staff to the scene at Bishopdale New World, said just 16 per cent of cardiac arrest victims ever leave hospital.
People should "just give [CPR] a go", he said.
"We really just need people to get these patients on their back, recognise that they're in cardiac arrest, and perform chest compressions. Even if it's wrong, it's still better than nothing being done at all."
St John has a free CPR app with instructions on how to use an AED and perform CPR.
You can also learn to perform CPR through a free course with St John.