While it is not unusual for work to be outsourced to Mercy, the private hospital has not performed public cardiac surgery before.
It is understood there is a backlog of cases at Dunedin Hospital, where the intensive care unit is being upgraded, and less space to manage patients after surgery.
Mercy Hospital clinical services director Philippa Pringle said the first two procedures were carried out last week.
''When a waiting list increases the way cardiac surgery has, then I think it's important for us to support the public sector,'' Mrs Pringle said.
A statement from Southern District Health Board patient services medical director Richard Bunton, who is also one of the surgeons performing cardiac surgery at Mercy, said the hospital was ''partnering'' with Mercy over the next year while the ICU was upgraded.
''Patients will be kept informed . . . partnering with private providers is common in other centres.''
Dunedin woman Sally Hume, who is waiting for cardiac surgery, said there was pressure on the service.
''My cardiologist said, 'We'll get you in somehow but there are people in the ward who have already had heart attacks and they have waited three weeks and they still haven't had their surgery'.
''I have had heart failure in April last year, and then again in April this year,'' Ms Hume said.
