Doctors are urging parents to ensure their children are fully immunised against measles because of an outbreak in Sydney's south-west.

There have been 40 reported cases in the area, with 10 sufferers being taken to hospital.

Dr Stephen Conaty from the South Western Sydney Public Health Unit says in all cases those infected were not fully immunised.

"So all the cases we're seeing are either in unimmunised infants because they may be too young to be immunised because you get immunised at 12 months of age, or in teenagers, so young adults who for one reason or another they've missed out on either one or both of their MMR immunisations," he said.

"If they do seem to be missing a dose or you really can't remember if they've had a dose, then you really should go to your GP.

"The vaccine is free and just say, 'look, I think I need another measles vaccine for my son or daughter'."

Opposition health spokesman Andrew McDonald is a practising paediatrician and says he has seen children die from measles.

"Now is the time for every parent to check their Blue Book to make sure that their child has been fully vaccinated," he said.

Dr McDonald is also reminding teenagers to have theirs too.

"Many children in high school missed the second needle at age four. It's never too late to get catch up."