An internal investigation will be conducted at Bankstown-Lidcombe Hospital, in Sydney's south-west, after a number of babies were given ineffective hepatitis B vaccines.

The acting director of population health for the South Western Sydney Local Health District, Dr Stephen Conaty, said 282 babies or mothers were given the inadequate vaccines.

"The fridge in the birthing unit of the perinatal ward was cycling to a low temperature and we think those vaccines are not going to do their job," Dr Conaty said.

"We think it's a problem with overstocking that caused the fridge to cycle to low temperatures."

The hospital has begun contacting mothers who gave birth between November 29 last year and January 22.

Dr Conaty said some children had to be revaccinated.

"There's a group of six infants that were immediately revaccinated because they were within a seven day time period of their birth.

"For most of the rest, we've taken the decision that they will be adequately protected by the the hepatitis B vaccine that they receive in their normal childhood vaccinations at two, four and six months."

External investigation needed: Opposition

Dr Conaty said immediate measures were taken to rectify the problem with the fridge and an internal investigation has been launched into the episode.

"The error, of course, is something that Bankstown Hospital and the local health district has to take responsibility for," Dr Conaty said.

"But, basically, this is a system error and, to my knowledge, everything has been done according to policy and procedure.

"We're using our normal, internal procedures to review this event and it's my personal view that that should be wholly adequate in this circumstance."

However, Opposition health spokesman Walt Secord said that was not sufficient.

"An independent external review [is needed] ... there is no point having NSW Health investigating itself," Mr Secord said.

"The newly installed health minister must guarantee that all protocols and procedures have been followed involving this bungle.

"I'm not a doctor and I don't know the nuances of vaccinations, but I do know that parents need to be reassured that all procedures and protocols have been followed."

Three women who received a postnatal dose of whooping cough vaccine this month have also been contacted and offered a replacement dose.

Bankstown-Lidcombe Hospital has been plagued by several issues in recent years.

In July 2016 a problem with an oxygen supply outlet left one baby dead and another injured.

It followed an incident at the hospital in January 2014, in which another newborn baby was affected by a loss of oxygen.