Canterbury District Health Board says the number of confirmed cases of measles in the region has now risen to 34, with a further five cases also being investigated.
In a release today, Canterbury medical officer of health Dr Ramon Pink said another four cases were confirmed this week and that they are concerned that numbers "are tracking upwards".
"Twenty-two of our 34 cases have occurred in people aged 28 and younger.
"This group is particularly susceptible to measles, and we need to increase vaccination uptake amongst this group if we're to stop the spread of measles."
Dr Pink said General Practitioners are now focusing on those who need vaccine the most - people aged between 12 months and 28 years who have never been immunised.
Some cases had been spread to people unknown to original carrier, he said.
"In most cases we've been able to identify the confirmed case who has spread measles to contacts in the same household, school or workplace ... but for some cases we haven't been able to identify any links, indicating that measles is spreading in the community.
"We're asking people who are unwell with measles-like symptoms, or who have never been vaccinated, to stay away from large gatherings.
"There is enough for our community to be dealing with right now without adding more illness."
Measles symptoms are a cough, runny nose or conjunctivitis, a fever above 38.5 degrees and a rash.
Those who think they may have been exposed, or are experiencing symptoms should call their GP before visiting.
More information is available on the Ministry of Health website here.