A Christchurch child has been diagnosed with measles, prompting the Canterbury District Health Board to urge parents to immunise their children.

The child had been given a first dose of measles vaccine but had not yet had a second dose, due at four years of age.

Dr Daniel Williams, Canterbury Medical Officer of Health, said advice had been given to parents at the two preschools the child attends.

"The advice urges parents who have not immunised their children to do so. People who choose not to vaccinate their children against infectious diseases are putting not only their own children at risk, but also other people's children," he said.

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The source of the child's infection is unknown.

Dr Williams said the fact the child received one dose of vaccine has helped reduce the severity of symptoms to just a mild illness.

He said the measles vaccine is highly effective, but no vaccine is 100 per cent effective all the time.

Measles starts with fever, runny nose, cough, red eyes, and sore throat. It's followed by a rash that spreads over the body. Measles virus is highly contagious and spreads through the air through coughing and sneezing.

New Zealand's target for immunisation coverage is 95 per cent of children fully immunised by eight months and then two years of age.

Canterbury has 93 per cent coverage for eight month olds and has achieved the 95 per cent target for two year olds.