A ground-breaking trial at Australian hospitals could double the time that a donor heart transplant is viable outside the human body.

The amount of time a donor heart remains viable for transplantation outside the body could double from four to eight hours, following a ground-breaking trial of a new device at Australian hospitals.

Cardiac specialists at The Alfred in Melbourne have performed five human heart transplants using a "hypothermic ex-vivo perfusion" to preserve a donor heart during transit.

In a recent heart transplant, performed on a Melbourne man, the donor heart was viable for more than seven hours - the longest time recorded worldwide.

The time extension allows donor hearts to travel longer distances and may lead to 15 per cent more heart transplants each year, researchers say.

Donor hearts are placed on a Swedish-developed "ex-vivo perfusion machine", which pumps a liquid through the heart muscle to cool it down to eight degrees Celcius and keep it supplied with oxygen.

"If the trial demonstrates that the donor heart is better protected with ex-vivo perfusion, as opposed to ice slush in a cooler, it could mean all donor hearts are transported using ex-vivo perfusion, no matter what distance," Alfred cardiothoracic surgeon David McGiffin said.

There could also be benefits for a patient's recovery. Trial participants have recovered well due to the support and oxygen given to the donor heart.

"The best chance that a patient has of surviving heart transplantation is if they come out of the operating room with a well-functioning heart despite a very, very long ischemic time, and that's what we think this system will deliver," Alfred director of cardiology David Kaye said.

The breakthrough comes after four years of research led by the Critical Care Research Group in Brisbane, with five sites across Australia and New Zealand involved in the trial.

The Alfred is the first to perform five transplants, with other trial sites including the Royal Children's Hospital in Melbourne, St Vincent's Hospital in Sydney, Fiona Stanley Hospital in Perth and Auckland City Hospital in New Zealand.