Northland health authorities are urging people to check they're protected from measles before travelling overseas.
A woman is in intensive care in Whangarei hospital with measles complications, after a trip to the Philippines where the infection is rampant.
Dr Virginia McLaughlin said the young woman who was unvaccinated, was admitted on Saturday with seizures and encephalitis, swelling of the brain.
Her condition is now stable.
"She developed a rash, then unfortunately this progressed to more serious complications. Five days before the rash appears, and five days afterwards, people are infectious."
Dr McLaughlin said the Public Health Unit in Whangarei had traced more than 100 people the woman had contact with during that period.
But the unit also wanted to hear from anyone who was at a netball game she played in, at Kensington Stadium on Friday evening, 10 days ago.
"It was a match between a team from Whangarei and a team from Dargaville, so we've managed to contact the people in both teams. It's just really a matter of anyone else who happened to be in the stadium at that time. It would be great if they could make contact with us so we can assess their risk."
Three of the four cases of measles in Northland this year had been contracted from outside the region, Dr McLaughlin said.
"If you are travelling outside Northland it's really important to check your vaccinations are up to date - that will protect the community and you, if you happen to be going overseas."
In 2019 there have been outbreaks of measles in Canterbury (now declared over), Auckland and Bay of Plenty. Cases have also been reported in the Waikato, Lakes, Capital and Coast, and Southern DHB areas.
There have been 149 confirmed cases reported nationally up to 22 May, with another 44 cases under investigation.
People worried they might have been exposed to measles should call the Public Health Unit on 0800 600 720, rather than go to a hospital or GP clinic, the DHB said.