Data from Ambulance Victoria shows more bystanders are performing CPR on people with cardiac arrest - improving their chances of survival.

Almost two-thirds of Victorians say they will perform CPR if they see someone in cardiac arrest, a good Samaritan intervention which doubles a victim's chances of survival.

Some 64 per cent of onlookers attempted CPR when they witnessed someone collapsed from cardiac arrest during the past financial year, according to the Victorian Ambulance Cardiac Arrest Registry annual report released on Friday.

That's up from 35 per cent in 2005/06.

More people also know how to use portable defibrillators than a decade ago, which has further improved the likelihood of a person surviving.

"You're more than twice as likely to survive a cardiac arrest now in Victoria than previous years," lead Victoria Ambulance researcher Karen Smith said.

Related CPD training: Basic Cardiopulmonary Resuscitation (CPR) – Developed by LearnEM

In 2014/15, bystanders used Automated External Defibrillators - often found in public places - 70 times on patients in a shockable rhythm, compared to just 10 times in 2005/06.

A shockable rhythm is the cardiac rhythm most favourable to survival.

Dr Smith says early intervention by bystanders greatly improves the "chain of survival" of cardiac arrest patients because minutes matter.

"Bystanders play a key role in that link because bystander CPR maintains the heart in a rhythm that's likely to be able to be treated by paramedics when they get there," Dr Smith said.

People who have no CPR training should still give it a go with the help of a Triple 0 operator who will explain how to do it over the phone until paramedics arrive.

"There's a saying that 'any CPR is better than no CPR'," Dr Smith said.

"You really can't hurt a patient in cardiac arrest. Any care is better than none."

Ambulance Victoria responded to 5657 cases of cardiac arrests in the 12 months to June 30, 2015.