The number of people catching mumps in Auckland is nearly four times higher than this time last year and the Auckland Regional Public Health Service says it's because fewer people are immunised.
The number of people diagnosed with mumps this year is 130 compared, with 35 last year.
In 80 per cent of the cases the sufferers were not fully vaccinated.
The ARPHS says it's concerned not enough young Aucklanders are immunised.
Seventy per cent of cases of mumps are seen in 10 to 19-year-olds and it can spread quickly in schools among those who aren't immune.
They say although "most recover from the disease, a number of people have suffered severe complications."
In men these can include swollen and painful testicles, sometimes leading to infertility, and some women can experience inflamed ovaries.
Pregnant women who are not immunised can suffer a miscarriage.
Dr Julia Peters the Clinical Director of the ARPHS says she is "urging parents to organise vaccination quickly" saying "we are in the midst of an outbreak."