The number of people getting whooping cough is on the rise and health officials fear another epidemic is on the way.
In the past three months, 340 people have contracted the disease, a five-fold increase compared with the same period last year.
Immunisation Advisory Centre director Nikki Turner fears it is the beginning of another epidemic.
Dr Turner says the rise is partly due to parents simply not immunising their children against the disease or vaccinating them late.
That increases the risk of hospitalisation four or five-fold.
The Health Ministry is alerting GPs to the rising number of cases, and says it has sent messages to parents encouraging them to immunise their children.