Fifty thousand Australians are at high risk of stroke because of dangerous undiagnosed heart rhythm problems, says a cardiologist who believes he has found a solution.

University of Sydney Professor Ben Freedman says a cheap iPhone-based ECG system will help identify people before they become ill.

He has told colleagues at a Cardiac Society of Australia and New Zealand scientific meeting in Queensland that the system is accurate and can be operated without special skills.

Called the AliveCor Heart Monitor for iPhone, it was invented by an Australian scientist and a US cardiologist.

Prof Freedman and his colleagues tested the device on randomly selected people aged 65 or older at 10 Sydney pharmacies.

As they expected, they found 1.5 per cent the 1000 people tested were unaware they were at risk of stroke.

He believes thousands of stroke cases could be prevented if the device is routinely used. He is now testing it at GPs' rooms, where it is operated by receptionists.

"Atrial fibrillation is the most common heart rhythm problem and is responsible for almost one third of all strokes," he says.

"The good news is that stroke is highly preventable with anticoagulant medication, such as warfarin, which can reduce the risk by 66 per cent."

"However, people with atrial fibrillation face up to a five-fold increased risk of stroke, and tend to have more severe strokes."