Genomic sequencing recently found that five cases of tuberculosis in Sydney were the same strain and genetically identical, with four of them overlapping at St Vincent's Hospital.

A statement from St Vincent's on Friday said it was likely one person had transmitted the infection to two other patients and one staff member while in hospital.

The person was not known to have tuberculosis at the time they were potentially infectious.

The hospital is working with NSW Health and clinical experts to identify a "very targeted" group of staff, patients and members of the public who may have been exposed, and is organising testing as a precaution.

"It's important to know that the risk of infection to those that we have identified as potentially being at risk is low," St Vincent's respiratory physician and tuberculosis specialist Anthony Byrne said.

"It's also important to note that if infection with TB does occur, most (90 per cent) people will never get sick and cannot infect other people."

The potentially serious bacterial disease most commonly affects the lungs and can be spread when infected people cough and sneeze.

The hospital has set up a hotline - 1800 943 123 - and further information is available at www.svhs.org.au

It said those who had been infected were receiving support and treatment.

Australian Associated Press