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 the hospital on Friday said the person likely to have transmitted the disease 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 established two enhanced tuberculosis systems to ensure it has the capacity for rapid testing.

They are prioritising people based on their risk of exposure, and whether they have underlying illnesses or are immuno-compromised.

A hotline - 1800 943 123 - has been set up and further information is available at www.svhs.org.au

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

Australian Associated Press