A mobile app has been launched in Victoria that will alert off-duty paramedics and professionals to nearby medical emergencies, potentially saving many lives.

Whether they're out with friends, walking the dog or at home in their pyjamas, off-duty paramedics in Victoria could be called on to save a life in a flash thanks to a new mobile app.

The GoodSAM application, launched on Tuesday, pings paramedics, firefighters, doctors and other trained professionals who are close by when a medical emergency strikes.

Off-duty professionals signed up to the app can accept the alert and provide first aid before emergency crews arrive.

It will focus on time-sensitive medical emergencies, particularly heart attacks, and Ambulance Victoria believes it will create an "army" of trained, life-saving good Samaritans.

"It's about ensuring we can get a trained bystander to someone in cardiac arrest as quickly as possible, because minutes count," Ambulance Victoria's chief executive Tony Walker said.

"An army of life savers, if you like, who are out there able to respond and support our paramedics."

Mr Walker believes cardiac arrest survival rates will grow "exponentially" once the system is fully rolled out.

GoodSAM uses location data to alert up to three trained bystanders within a short distance of the patient - 400 metres in Melbourne and five kilometres in country Victoria.

The launch follows a successful four-month pilot in which two patients were successfully resuscitated during cardiac arrest.

Keith Young, 52, counts himself lucky after his neighbour, paramedic Darren Murphy, rushed to assist when he suffered a heart attack in February.

Mr Murphy accepted the alert on his phone and swiftly rode his pushbike to Mr Young's house to perform CPR before ambulances arrived.

"My family have said they will be forever grateful for the authority, confidence and calm that Darren portrayed in an unimaginably terrifying situation," Mr Young said.

About 1000 paramedics are signed up and more than 100,000 first responders could eventually be involved.

Responders will be trained on how to introduce themselves and Victorians shouldn't be shocked if a stranger arrives to help looking like they've just jumped out of bed.

"We've had two of our paramedics actually respond in their pyjamas," Mr Walker said.

Premier Daniel Andrews said the app matches modern technology with skills and good Samaritan spirit.

"Minutes matter, seconds matter, in terms of saving lives," he said.

GoodSAM is already used in the UK but the Victorian launch is an Australian-first.

Firefighters, doctors, nurses and physiotherapists are among those who can register.

Victoria Ambulance says integrating the system will cost about $800,000.