Eleven cases of measles have been confirmed in Waikato and a further 13 cases are under investigation.
Medical Officer of Health Richard Wall said five of the suspected cases are people who attended a Tainui Waka Kapa Haka Festival two weeks ago , which was also attended by a person already infected with measles.
The incubation period for measles is about two weeks, so most people who were at the festival will have become unwell by now if they were going to get the virus.
But a few could still get it this week, Dr Wall said.
Measles usually starts with a fever, cough, runny nose and eyes, followed by a rash two to five days later.