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.