A police prosecutor in Christchurch has been diagnosed with measles.

Photo: 123rf.com

The Law Society has issued a notice to its lawyers and staff in Christchurch, and is warning that the prosecutor has likely come into contact with many people while infectious.

Canterbury District Health Board said there's now 25 confirmed cases of the highly infectious disease, with the number likely to rise.

Up to 100,000 Cantabrians need to be vaccinated and there's concerns vaccine supply levels.

The society said it has also notified the Judiciary, all Ministry of Justice Staff, all police staff, representatives of the profession and all ancillary agencies who work within the courts.

It said the number of other contacts would be extensive and impossible to trace.

Anyone not immunised, or only partially immunised who have been in contact with the prosecutor or any of their contacts, was at risk of catching the disease, the society said.

Health experts are warning that for everyone in Canterbury to be protected against measles, 95 percent of people would need to be fully immunised against it.

Immunisation Advisory Centre research director Helen Petousis-Harris said it was a huge undertaking for clinics to be assessing potential cases of measles, while vaccinating hundreds of people.

But she said the level of immunisation compliance needed to be high to ensure everyone in the community was protected.

Prime Minister is urging the public to ensure they are fully immunised against the virus.

"This is just another strong message, people cannot rely on others being immunised to protect them.

"We need to keep encouraging people to take up immunisation."

Measles facts: