The West Australian Health Department says it has taken steps to try to stop the spread of a measles outbreak at a Perth private school attended by up to 200 children who have not been immunised.

Perth Waldorf School — a Steiner school in the southern suburb of Bibra Lake — confirmed a high school student had been diagnosed with measles after becoming infected while holidaying in Europe.

There are now fears he may have infected fellow students at the school, where a Health Department spokeswoman said as many as half of the 400 pupils could be unvaccinated.

Measles is generally well contained through an established immunisation program in Australia, but if a person is not immunised then serious cases of the disease can be potentially life-threatening.

The Health Department released a statement saying it was powerless to force parents to immunise their children.

"However WA Health has advised that the school is to exclude all exposed unvaccinated contacts from attending school until there is no longer a risk of them developing measles and exposing additional school children," the statement said.

"Measles-mumps-rubella vaccine is available for free to anyone not already vaccinated aged 19 years and under through the National Immunisation Program.

"WA Health has made an offer to the school to vaccinate students through specially convened vaccination clinics."

The school's administrator Jean-Michel David was unavailable for interview, but released a statement saying the department's advice had been followed "to minimise spread of infection to the community".

"Perth Waldorf School is continuing to work closely with the WA Health Department and the family and student involved in this one confirmed measles case," he said.

"The communicable disease guideline for measles stipulates that all unimmunised and non-immune individuals who have been in contact should be excluded for a period of 14 days from the onset of the rash.

"The school had put this in place as soon as it was notified by the department of the measles being contracted by the student.

"The same guideline also stipulates that vaccinated or previously infected contacts are not to be excluded.

"We are acting on the advice of the Health Department with regards to how to best manage the situation, including communicating information with the community."

Mr David did not say whether the school would accept the department's offer to conduct specially convened vaccination clinics.

Perth Waldorf School includes a playgroup for two-year-olds, a pre-school and a primary school.

The Education Department Act states that schools in WA can ask for vaccination records of children during enrolment, but can not reject applications based on vaccination status.