Almost 600 women and nine babies may have received ineffective vaccines after a faulty refrigerator thermostat at Royal Prince Alfred Hospital went undetected for more than a year.
The hospital is contacting all mothers admitted to its postnatal ward 8 East between August 2013 and January this year "as a precaution" after the fault was detected earlier this month.
The problem meant routine vaccines, including Boostrix (a diptheria, tetanus and whooping cough combination), another for measles, mumps and rubella and a third for Hepatitis B, may have been exposed to temperatures outside the optimum storage range "and their effectiveness cannot be guaranteed," the hospital said.
The hospital's general manager Deborah Willcox said there was no risk associated with receiving the potentially less effective vaccines.
"However, these vaccines may not provide adequate protection against some illnesses," Ms Willcox said.
Health Minister Jillian Skinner said the failures at the hospital were "disappointing" and she had asked the department to ensure the error was not occurring elsewhere.
"A full investigation is underway to guarantee all public hospitals are compliant with medication storage protocols," Ms Skinner said.
"I'm advised measures will be introduced at RPA to ensure this incident does not happen again."
A free clinic has been set up at Croydon Health Centre for the affected patients, many of whom were expected to need revaccinations.
"But in some circumstances, they may just have a blood test to check their immunity or they may actually decide that they don't need anything further done," Public Health Unit Clinical Director, Leena Gupta, said.
The mothers of nine babies administered with the potentially ineffective Hepatitis B vaccine were also being contacted, Dr Gupta said.
A hotline, 1800 300 243, has also been set up for anyone with concerns.
Ms Willcox said there was also no evidence to suggest that any of the patients had contracted the illnesses they were supposed to be immunised against.
The investigation would seek to determine why the problem remained undetected for so long, she said.
"On behalf of RPA, I would like to sincerely apologise for any concern this incident may have caused, or may cause," Ms Willcox said.
No other vaccines had been affected, she said, and the public could "continue to have every confidence in Royal Prince Alfred Hospital".
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