The number of people with the mumps in Auckland has hit 1000, with experts saying it now can't be contained without vaccinations.
The latest Auckland Regional Public Health figures show 1003 people have contracted the disease since it broke out earlier this year
And the more people that have the disease, the faster it travelled through the community.
A senior lecturer in vaccinology at Auckland University, Helen Petousis-Harris, said it had reached a threshold where it would keep spreading until it ran out of people who could contract it.
"The only way at this stage that you can prevent further spread is to reduce your susceptible people and the only way you can do that is by vaccinating them," she said.
Those most likely to catch the disease were the so called "lost generation" of 11 to 29 year olds who had low vaccination rates.
Estimations varied but up to 40 percent of that group may be susceptible, Ms Petousis-Harris said
The Auckland Regional Public Health service was not yet calling the outbreak an epidemic but Ms Petousis-Harris said it fitted the definition.
"When you just get a few cases, perhaps clustered in a small geographical area it's generally considered an outbreak. Once that becomes wider - and there's not an exact line in the sand - then you move into an epidemic situation."
Free vaccinations were already being offered in Auckland.