Doctors are treating unimmunised New Zealanders in carparks and outdoor areas in an attempt to reduce the spread of measles, as the number of confirmed cases of the contagious disease rose to 25.

A severe measles outbreak, described as the worst “in years” continues to spread in the Canterbury region of the South Island, with health authorities saying 100,000 people need to be urgently vaccinated, and more cases of the highly infectious illness are expected in the coming days and weeks.

As a result, doctors have moved their clinics outside, to reduce the chance of the disease spreading in confined surgeries and waiting rooms.

Vaccine supplies have run out in the affected region and tens of thousands of extra supplies are being flown in from around the country, as doctors use their last stocks to vaccinate those most in need of protection, including young babies and those with other health issues.

“It can now be assumed that measles is circulating widely in our community,” said Ramon Pink, Canterbury medical officer of health.

The minister of health, David Clark, said government health officials were closely monitoring the situation in Canterbury to try to prevent the disease spreading to other parts of the country.

Government health representatives were on the ground in Christchurch, and said they would issue an update on Wednesday regarding the latest steps that are being taken to isolate the outbreak, and ensure it does not spread beyond Canterbury’s borders.

People who were born before 1969 or have had two shots of the MMR vaccinations are considered fully protected, Pink said, but those aged between 29 and 50 who have only had one measles vaccination are not considered immune, and will require an additional shot.

New Zealand, like many other countries, is seeing a resurgence of measles, following scepticism from growing sectors of the population that the MMR vaccination can cause autism – a belief that has been thoroughly and repeatedly debunked by global health experts, including the World Health Organisation.

“The resurgence of measles is of serious concern, with extended outbreaks occurring across regions, and particularly in countries that had achieved, or were close to achieving measles elimination,” said Soumya Swaminathan, the deputy director general of programmes at the WHO. “Without urgent efforts to increase vaccination coverage and identify populations with unacceptable levels of under- or unimmunised children, we risk losing decades of progress in protecting children and communities against this devastating but entirely preventable disease.”