Children exposed to measles at two preschools in Christchurch are being contacted as Canterbury health authorities continue efforts to contain an outbreak of the disease.
Two cases at preschools emerged this week: 54 children in total exposed to the disease.
Canterbury's Medical Officer of Health Dr Cheryl Brunton says 51 of these have been contacted: half were not immunised.
Dr Brunton says the children have received immunoglobulin, which provides a temporary boost to the immune system. They are allowed to return to preschool.
Children who have received one of two measles, mumps and rubella vaccinations are also able to return.