Health experts say the majority of the measles cases in Northland, Auckland and Bay of Plenty are likely to all have stemmed from the same initial patient.
So far this year, 137 people across the country have been infected.
The Institute of Environmental Science and Research said until now, they haven't been able to establish a link between the cases.
But Jill Sherwood from the institute said more testing has shown it is likely the spread of measles in three of the regions are connected.
"What that underscores really is the danger of even one importation," she said.
"If vaccination rates are low enough and...enough susceptible people come into contact with that first person...then it will continue to spread."
Ms Sherwood said there are also concerns that the number of cases this year could overtake the 2014 outbreak of 280 people infected and 61 hospitalisations.
"We've still got ongoing transmissions in both the Bay of Plenty area and Auckland - particularly in Waitemata, you've got big population groups there so it is a concern," she said.