The patients both contracted Mycobacterium chimaera (M. Chimaera) from a surgical heater-cooler units at Prince of Wales Hospital.

The particular brand of units, manufactured overseas by Sorin, have linked to over 70 cases of the infection internationally.

NSW Health issued several alerts - the first in August - advising open heart surgery patients to see their doctor if they had undergone the procedure in the past five years.

The second NSW case is a man in his 40s. A woman in her 80s also contracted the infection, NSW Health said. The woman is now recovering. There is no risk of affected patients passing on the infection to their families, friends or the general public.

Prince of Wales Hospital is one of four public hospitals that use the heater-cooler units.

The contaminated equipment was removed from Prince of Wales and St George Hospitals, as well as Sydney Children's Hospital at Randwick and the Children's Hospital at Westmead as a precaution in August when NSW Health learnt of the risk from international authorities.

The units were also used in a number of private NSW hospitals and hospitals in other states and territories, including Queensland where the first M. Chimaera case in Australia was detected.

The units, which control the temperature of the blood during the procedure, transmit the infection to the formerly sterile surgical area and the heart's new implanted valve and graft. Investigators suspect the units were contaminated during their manufacture.

Chief Health Officer Dr Kerry Chant said NSW Health was watching for further cases at the Prince of Wales Hospital, after international clusters of M. Chimaera infection suggested that there was an increased risk to other heart surgery patients at facilities where an M. Chimaera case had been detected.

A total of 70 confirmed cases worldwide have been identified in patients who had  undergone open heart surgery in which the contaminated equipment was used. The infections were identified between three months and five years after surgery.

The first case was in detected in Switzerland in 2012.The first Australian case was detected in Queensland in 2016.

Several independent studies reported open heart surgery patients had developed post-operative prosthetic-valve endocarditis caused by the mycobacteria.

Symptoms of the infection could include fever lasting more than a week, pain, redness, heat, pus around a surgical incision, night sweats, joint and muscle pain, loss of energy and failure to gain weight, or failure to grow in children.

Dr Chant said NSW Health sent letters to patients who underwent open heart surgery between January 2012 and August 2016 informing them of the risk, symptoms and what to do if concerned.

"We also contacted private hospitals in NSW and have been advised that private hospitals in NSW that used affected equipment have also sent letters to their patients, informing them of the risk," Dr CHant said.

"Patients have been asked to watch for M. chimaera symptoms – persistent fevers, increasing or unusual shortness of breath, and unexplained weight loss," she said.

NSW Health has set up helplines for patients seeking further information.

Dr Chant also urged GPs and relevant specialists to go to the NSW Health website for the latest information.

In August NSW Health assured the public that the infection was rare and risk to patients was very low and there was no ongoing risk in NSW public hospitals.

Infectious disease specialist at the NSW Clinical Excellence Commission, Dr Kate Clezy said "the risk of infections to an individual patient is very small, but it's important that we've alerted clinicians to the risk and put systems in place to reduce the risk further."

NSW Health formed an expert panel of clinicians and representatives from the Clinical Excellence Commission, chief executives of Local Health Districts and Health Protection NSW once it was alerted to the potential infection risk.

The contaminated units were either cleaned and verified as clear of contamination or have been replaced with new units, NSW Health said.

A safety notice was issued to public and private health facilities on July 8, and updated on August 4, to notify clinicians of the very low risk of infection, the department said.

NSW Health and other jurisdictions are working with the Australian Commission for Safety and Quality in Health Care to develop a national infection control guideline on minimising the risk of infection relating to the use of heater-cooler units.