An Auckland vaccinologist is urging people to check their likely immunity to mumps as the number of people who have contracted the disease tops 1000.
The number of victims is the highest since a similar mumps outbreak in the 1990s.
The more people that have the mumps, the faster it will spread through the community.
An Auckland University senior lecturer in vaccinology, Helen Petousis-Harris, said the current spate matched the definition of an epidemic, even though it was officially called an outbreak.
Most of those infected are in the 11-29 age group where vaccination rates could be as low as 40 percent.
Ms Petousis-Harris said the rate of infection wouldn't die down until the number of susceptible people dropped.
And she said immunity from vaccination wanes over time.
She said people who did not have mumps as a child or who suspected they were not fully immunised should get one of the free jabs being offered in Auckland.
"Really think about whether you, or your whānau, your family have received two doses of vaccine or have had the mumps and if not then seek out receiving appropriate doses of vaccine, because really that is the best way to prevent this from transmitting further."